Don't know of any such program, but discussion of such things is not allowed. It's against xbox rules or something.
Fallout 3 for the 360 has no way to actually enable any mods you download, right? The DLC is handled through XBL IIRC.
So even if you
could get mods onto a 360's HDD, you couldn't actually use them.
So if you want mods (and really, with mods like Lexx's classic Advanced Power Armor, Enclave Commander, and MMM how can you
not want mods?), it's PC or bust.