He can, but not the way that Van Horn or McKie did.
Van Horn may have been "retired," but Dallas still owed him money, and they still owned his contract. Basically what happened there is that Cuban told him that he could stay home and sit out the remainder of his contract. Same deal with McKie.
Williamson is no longer under contract. He's apparently (if nbrans' cite can be believed) not officially retired, but we'd have to re-sign him to trade him, and he wouldn't be under any obligation to sign with us in the first place.