You can assign them properties. The only two being WEIGHT and CONVERT_TO
Weight is a floating point number.
CONVERT_TO:ooo, ooo is an object Unique Identifier.
Example
ADD HEAVY_ROCK CLONES=(MISC_BOULDER) PROPERTIES=(WEIGHT:5000) NAME=(ROCK)
Try adding this into a dungeon as an amusing ninja training tool.
Quite easy to kill yourself by throwing it to much.

I haven't experimented with CONVERT_TO - I imagine that this is what
you use to decide what an object does when its charge runs out. (e.g Storm ring)