Yes, I see. That one can use to set the groups.
I'm looking for a convenient way to specify them.

But that is maybe a little difficult.

So I fear I have to do this on my own.

I have found freeglut and wxwidgets, I understand one can use those, even in commercial things.
So I think I'll make an own group editor and plug this into poser. The geometry information might be passed with files or with a socket. Ah, it sounds overkill as I am writing these lines :(