thats pretty hard to make...imo just skip this idea. Even if you gonna modify functions like canDoCombat, canSee~, isSightClear~ to allow multifloor hits, its really hard to realize when monsters are able to hit and when they are not. Basic thoughts are about:
-if floor > 7 - false;
-if attack is melee - false;
-if got a tile above creature - false;
erm, just skip this, thats pretty hard to make it bug free imo...