Since I wanted to stick a DUPLO brick to the feet of other objects, I modified the scad file with an extra 4th parameter (nibbles on or off)

First try was: I imported models to Meshlab and killed the caps of the brick. A dumb way to it as the commenters pointed out :)

This thing gave me the idea for a larger project: http://www.thingiverse.com/thing:2106

More of the story is here: http://edutechwiki.unige.ch/en/Meshlab_for_RapMan_tutorial and http://edutechwiki.unige.ch/en/Sketchup_3D_printable_objects_tutorial