If you're lucky you'll drop Bag of seed from undead mob but this bag drop Mystical seed so you can farm mob and break the pack !

Bag of seeds, like others loots, can be edited (in 'config folder' in latest versions, in 'save folder' in the past).
Seeds can be edited with the item tag 'tombstone:seeds'.
Additionally, the bag of seeds only drops if the player kills an undead himself (not with a farm, or it's the 'farm' mod doing this by adding a fake player 'non conventional').