Yes, you can send the MIDI to external MIDI hardware, using the “MIDI Instruments” section of the VST panel.
You could also use a more lightweight sample library, like NotePerformer. But, if 10Gb is going to ‘break the bank’, then realistically you need to look at additional storage rather than reducing your data.
And realize that, if you have only a little more than 10GB on your boot drive, you can use the Library Manager utility to move the Dorico sounds to another drive once you download them.
And to add… that if you are concerned about 10GB, then it is a warning sign that your machine may cause you more frustrations than most. Speaking as a person who was stubborn about upgrading 
Dorico may indeed run, but I do not regret the extra bit of investment to let it have enough resources to run extremely well.
Hi thanks for the replies! 
I must admit it’s more of a principle thing. (I have space enough and my machine is working well)
I own already many libraries and yes, I can use library-templates to load vst’s into Dorico. But I’d rather just use Dorico in a light version (like I used Sibelius) and have it open next to Cubase where I already have loaded many instruments into memory.
I think that is a reasonable reason for use of music notation software in an as light as possible way.
Honestly, you should use NotePerformer. Best solution for quality output with as little effort as possible and light footprint on your system.
I don’t think that is Karel’s concern. It is not that he couldn’t use something smaller; it’s that Karel doesn’t want to load the HALion sounds. I don’t agree, but there it is.
Thanks @MarcLarcher, but yes @Derrek is right. I would like a light alternative that doesn’t sound good, but just gives me the pitches. For instance the midi sounds like in Sibelius.
This way my system can work optimally with the orchestral template I already use in Cubase.
I’ll use HALion because there is no other (free) alternative, but I find it strange that this isn’t on the wish list for Dorico development
I understand. I nevertheless think NotePerformer is a much better alternative, although not free, because it’s cheap but can really make a difference in proofreading (pizz/arco, slurs, dynamics … You really can hear the difference). This could explain why there’s no feature request like yours 
I, like a lot of others here, use Vienna Ensemble Pro to host my VST templates outside of Dorico. I don’t own Cubase so don’t know exactly what’s possible, but I seem to recall others mentioning that it could be used similarly. Is that a possible solution? Continue to use Cubase as a VST host, and have Dorico connect to it?
Cubase is not, strictly speaking designed to “host” instruments. Cubase users with large sound collections often use VEP to host sounds, as one can already do in Dorico if one uses the types of sound banks that make it worthwhile. (For example, John set up a version of Iconica for Dorico using VEP to host them.)
Hi @KarelB, how about following alternative : You know, there is also a Dorico SE Derivate which comes with a much smaller, GM-like sound set. So you could just download and install the SE content package and then in Dorico Pro go to the Play mode and apply the SE template, which would load much faster. The sounds are cheap, but as you said, that’s not a problem for you.
Thanks @Ulf , a good tip!