Depends on how many creeps/units you have in your map so you have to be patient. Double click a unit, then click Item Dropped then change Item Dropped on Death to Use Custom Item Table then there you will add the items you want to that specific unit.
But to prefer you how item dropping works on melee map, they must have random type items as drop. Lesser creeps with level 1-4 maybe should have Level 1-2 Random Miscellanous or Permanent Items as drops and greater creeps with level 5+ should have Level 2 and above Random Miscellanous or Permanent items as drops to scale with their difficulty.
You will have to do this for each unit in your map by the way to inform you.