Well, I don't think just anyone can. Kenzer got the rights to all 1e/2e AD&D and D&D material for their Hackmaster game - Greyhawk, Mystara, etc, etc, etc.
IIRC, on the d20 mailing list, Ryan Dancey mentioned it was a somewhat unique situation. Though I imagine it involved a lot of money as well.
Anyway, too bad it's for Hackmaster. I find that about as distasteful as I find SJG's Munchkin. (Except SJG doesn't have the rights to my beloved Mystara. But I digress).