I created multilayer 2D scenery with Adobe After Effects CS 5.5
Now I want to move a “virtual 2K HD camera” over the 5K scenery to get a moving shot. Is there any way to cut out parts of the picture with an animated frame except the virtual camera?
Your scene is as big as you built it.
The camera "sees" whatever dimension you make your comp to be (of course the lens you use matters as far as what you get in the frame, but the size of the frame is determined by the size of the comp).
so all you need is to set the comp size at 2k (or HD, not sure I understand what you mean by "2k HD camera").