the drum grid editor is self explanatory if you open it up. i'm not sure if the manual deals with it properly. as to the other instruments you may be able to enter them as midi manually using editable notation. midi is normally the solution when you need specific notes and timing. if you are using RTs for the rest of the track it might not 'match' exacly in terms of sound but with short sharp hits it will probably be fine. a bit laborious but easy enough.

with RB you can add extra tracks to work on and its all 'upfront' and visible and you can then mute the original RTs and bits you want the midi to take over fromn