Use the move command in the init of the unit to make it move in the building on the upper floors marked by a marker.
In steps place a marker very close to the building you want the unit to move into and name it "building1" , then name the nearest marker building "mb" by placing this code in the init of the unit .
mb = ((getMarkerPos "building1") nearestObject buildingID); this move (mb buildingpos 5);
Building ID is the ID of the building you selected and 5 is the position in the building (you should verify the number , its visible in game where it works).
As long as you start you should observe that the unit does not go to the upper floor but on the ground level.