Ah! I see. You want to be able to collapse a section of ground as if it just suddenly fell from beneath the invader's feet! They'd be running across your trap without even realizing it, when suddenly, BOOM, they're all five levels deeper in a pile of smoke and rubble.
Well, the problem is this: all z-levels in the game are basically divided into two parts, which are both capable of supporting infinite amounts of weight on a single tile's worth of material: a 'floor' section and a 'wall' section. Each single 1x1x1 square in DWAAARF FORTRESS is comprised of one of each of these parts, a floor section on which dwarves can walk with a wall section on top that prevents movement and creates a single floor section on the tile above it. Whenever a single one of these sections (or more, of course) is connected to any size clump of floors or walls suspended in midair, and any 'anchored' square (a tile somehow linked to the edges of the current map,) it will hold up those suspended tiles. So, you can end up with crazy stuff like this:
(Side view)
# = wall
_ = floor
#######
#### #
#### #
#### #
#
_______#____
The answer to your question, basically, is no. To collapse, a section must be 100% removed from the landscape. HOWEVER, by using 'supports' (walls that can be destroyed remotely by a lever,) you might actually be able to create a sort of a 'seam' around the area you'd like to collapse that could be walked across and detonated on command. Still, I'm not sure. I haven't used supports at all myself.