uhh somehow I have told the whole question in title. is it possible? I saw some cool animations like “sitting slavic” in a server called “cityrp”. are they made with bone modifications using lua or are they model sequences? I am %100 sure they work on every model.
I would guess that you can position the bones - so long as you know the names of the bones and how much you want to move them by (which could be specific to a given skeleton / armature).
You can do it either by using Lua (I think this functionality was added later on), or by precompiling an animation into the mdl file.