Is a sp possible (like sf)?
You can hack one together as a Playing Technique (search the forum; I’ve definitely explained how to do this before).
“sp” doesn’t exist because it’s not “like” sf. sf is an abbreviation of sforzando or sforzato, not subito forte (though this is frequently misunderstood, and sp does appear in printed music).
Playing techniques, create a new one, use glyphs.
If you want the dynamic reflected in playback, assign the dynamic and then hide it via the properties panel.
I would be confused if I saw “sp” in a score, and I would not interpret that marking as “subito piano.” So be careful!
Absolutely. I have probably read 100,000 pieces of music in my life and I have never, ever seen a dynamic marking “sp”.
The universal markings for an immediate change in dynamic are
subito p
sub p
(sub) p
I have never seen anything else.
I am just engraving a score from a russian composer, she defenitely wants to have sp (a lot of sp’s). The workaround with Playing Technique is possible, but not as elegant as with buildin dynamics (and much more work in engrave mode).
Someone could say, “I want the tempo marks to be displayed upside down, why can’t Dorico do this natively?” The answer is, of course, because it’s wrong.
Not blaming you. Engravers are often stuck needing to typeset what the composer or the publisher wants, even if it’s wrong. Seems like that’s where many of the workarounds are required. Composers are notorious for ignoring written conventions. Sometimes it’s intentional and thoughtful, other times it’s ignorance.
In this special case I can understand the composer. Subito p or sub p is to long, there is often not enough space and one need a short term. Anyway, engraving rules are a work of progress, especially in modern music. Who has the authority to say this is wrong or that is right?
Of course the composer is welcome to write any marks she wishes. In this case it would just require explanation in the score, since the mark doesn’t exist and is unknown to other musicians. And hence you shouldn’t expect Dorico to support it natively.
Dynamics are traditionally abbreviations of Italian words. There are precedents for multiple pianos or multiple pianos, mezzo fortes and mezzo pianos, sf or sfz or fz to mean sforzando/sforzato/forzato.
You’ve got four skilled musicians in this thread saying they don’t understand “sp”, because there’s no precedent for it in the (traditional) literature.
You’ve also got a highly-respected piece of notation software that doesn’t want you to write it because its developers also believe “sp” is wrong.
To ask you another question, if “sf” is short for “sforzando” then what Italian word is “sp” short for?!
(I repeat, “sf” is not short for “subito forte”!)
edit: I went away to see what Elaine Gould’s “Behind Bars” had to say about sp. Absolutely nothing, as it happens. Obviously there are plenty of examples of sf and p. sub but nothing explicitly stating that “sp” isn’t a dynamic.
I then went to Google. I searched for “sf dynamic” (without the quotes). Seven of the eight results on the first page were about musical dynamics. I then searched for “sp dynamic” without the quotes. One result on the first 10 pages (yes, I really mean that) was about dynamics, and that was https://en.wikipedia.org/wiki/Dynamics_(music) which doesn’t actually mention “sp” anywhere.
I’m not saying that I have the authority to say that “sp” is wrong, but I defy you to find it in commercially published literature from a respected publisher.
I am sympathetic to your problem. You are working on contract and the person paying the bills is asking you to do something that is not musically sensible and really not in the composer’s best interests, assuming she wants musicians to understand the music. So I am not asking you to pick a fight with her. You will just have to find a way to make her happy.
But maybe it is worth noting WHY composers use “subito”. After all, dynamics are intended to be immediate. ff doesn’t mean “start thinking about maybe getting louder if the mood strikes you.” It means “Play this here note loudly and keep on until I tell you differently.” Likewise, pp means “quiet right now.” In that sense, “subito” is actually somewhat redundant.
My view of “subito” is to inform the musician that we want a big change in the musical attitude – a change that is probably different than you would normally play it based on your musical training. In other words, “Subito p” tells me, “We’re all getting quiet suddenly for a great musical effect. Surprise the audience.”
And that brings me to another disagreement I’d have with your composer. Getting loud suddenly is really not at all like getting quiet suddenly. It is easy to get musicians to hit it hard, and there are many different ways to indicate this (accents, ffff, sf, szf, etc.) I don’t recall any music ever indicating subito ff. I suppose there are some examples out there. Every musician would understand that, but it just isn’t necessary. Musicians are happy to get loud without much encouragement.
I am no supporter of notational police, but it seems to me that using sf to mean subito forte is indefensible on the grounds that sf already means something else.
On the other hand, since sp does not traditionally mean anything, why not extend the musical vocabulary and use it for subito piano? I dont see why it has to be an Italian word (sporco?) to qualify as an acceptable musical term. A well-known score of Brahms II uses pf as an abbreviation for poco forte and nobody complains.
David
… and after having read this thread no one can ever say again ‘I have not seen a sp yet’
…only “I wish I had never seen…” 
I think that sp might be an excellent innovation. Claudio Arrau did something like that in his edition of Beethoven’s piano sonatas to show where he felt Beethoven intended a subito dynamic. He used small encircled italic s after the dynamic marking.
But there are problems until it becomes universally accepted:
The composer or conductor will have to explain it, taking up valuable and often expensive rehearsal time.
This composer will have to explain it in a footnote in all their publications or risk being misinterpreted.
The composer will risk the ire of performers whenever they are distracted by the unfamiliar marking or mistake it for a mp or sf or whatever.
For these reasons, many composers use innovative notation only when it is absolutely necessary and avoid reinventing the wheel. I think that they are right to do so. Composers have a hard enough time getting their music played and played well.
Dorico is able to create new noteheads, new playing techniques and so on. I wish in the future there will be an editor for dynamics 