I’m assuming he needs the interval to drive the sample library.
The cleanest way would be to add the note in another voice and hide it by setting its opacity (via the Color property) to zero and hiding its stem+ledger line (if applicable).
The “m2” defines the interval. You can show an auxiliary note if you want using the “Appearance” property, and there are several Engraving Options about how to display the auxiliary note.
If the sample library wants a 2-note chord to play a trill, that’s just weird. (But some sample libraries are just weird, if they were never meant to be used with notation software…)
It’s exactly how they work. Playing a minor second interval will play a half step trill and a major second a whole step. CSS also have different delays for different legato speeds triggered by different velocity ranges. It makes them sounding great when using them in the right way for mockups. But I think it makes them rather unusable for notation programs because the timing will go all over the place. There are also key switches triggered by different velocities to switch from sustain to legato.
It’s possible to replace key switches by CC58 but I don’t know if Dorico’s articulation maps support that. If so I’d recommend to use that and load the “Classic Legato” patches instead of the regular ones to avoid timing ups and downs and you can address half and whole tone trills directly.
If Dorico eventually allows notes to be indicated in Playback that do not show up as notation, then you may be able to accommodate CSS, but you would likely have to add each trill manually and separately.
With CSS writing trills can be tedious but the result is very convincing. It’s a habit to get into.
It’s possible to replace key switches by CC58 but I don’t know if Dorico’s articulation maps support that. If so I’d recommend to use that and load the “Classic Legato” patches instead of the regular ones to avoid timing ups and downs and you can address half and whole tone trills directly.