Is there any way to change a creatures eat item when it did not start with that one????
Is there a way to start an infinite drain that a creature does not start with????
I'd suggest that you make two creatures (or more, depending on whether or not you want the creature to have both of those attributes at once) that are exactly alike, except the first doesn't start with those attributes, and the second does. Then you just replace creature A (attributeless) with creature B (the one with attributes), and that should be it.