It would silly to assume that Tom Bombadil is the WitchKing and even sillier if he were Eru. But after reading the first few posts I realized that you(pl) were just being that: silly.
Tolkien could not or would not elaborate on Tom's origin, but I did glean somewhere that either in the Lost Tales or some other work that there were things in Arda that were older even than the Valar.