What I want to is turn several flyers (namely V-Wing, ARC-170 and Vulture Droid) into props. The problem is, that the models show these vehicles in their standby stance, but I want them in flight mode. What I tried is setting them up with ClassLabel = "animatedprop" and letting them execute the takeoff anim right away, but instead of staying in their flight pose the takeoff anim is looping infinitely

So, has someone succeded in doing this or is model manipulation in XSI or the likes the only way?

Flyers are modeled in their landed pose and you're going to have a problem with flyer animations because of how they work. The takeoff animation isn't a single frame, its a series of frames playing from start to end on takeoff and in reverse when landing. The only ways I can see you having a fixed flight pose is to export a new basepose or strip the models down to just geometry while translating parts into flight pose, both of which can be done easily in XSI.