Commander in Chief,
You can't do anything like that with EnAn. Well, actually, you can adjust the animation as it is playing with the Rotate Limb, Offset Limb commands, but that works with the standard DBpro animation commands too.
What GTA IV uses is called "Euphoria". It is a dynamic animation control system. Instead of making "animations" for your characters, the engine controls your models through behaviors. It is a mixture of animation, AI and physics that allow your models to react to situations.
I'd love to (at some point) work on a generic form of this functionality. Too much on my plate already though!
Ron

a.k.a WOLF!