Personally, I never got into the idea of that sort of thing for Pokemon games. Um, to clarify, hacking is to do with games that already exist, right? Not sure off the top of my head what making a fan-made game is called. Emulating?
Hm, okay. The idea is that Roscoe has a Leech Seed attached to himself (probably permanently) that would act as a reservoir of sorts for any excess drained energy, which would give him a reason to drain, or 'leech', other Pokemon more often. The stored energy could then be used either over time to heal Roscoe, or perhaps even take the place of real food for a time if necessary.
I have some reasoning behind how and why this would work. The energy/health that a normal Leech Seed drains in battle goes, more or less, through the seed and its vines in order to get to the recipient. Roscoe would basically be directing his drained energy through that same pathway, except it ends with the seed itself, and be able to reverse it.
Going by the games, a Grass-type Pokemon wouldn't be affected by Leech Seed. Since we're in a roleplay setting, it could be expanded a little, I think, to say that a Grass Pokemon could even manipulate a Leech seed a little in the manner I described. It wouldn't work with other types/elements, and I'd say that implementing such a thing is more likely to fail than to succeed.
...I realize that I pretty much branched away from items.