Only men or only married men? One of my favorite priests is a convert from the Presbyterian Church. Check out the archives for the Journey Home on EWTN's web site. There are priest converts from other denominations.
I thought there must be, but rarely hear of them - it seems mostly the Episcopalians make the news. I was thinking mainly of the married ones, as I know there would be less or no impediment for a single man to be ordained from any background as long as there is a true call. I think it should remain the exception, and if the spouse should die, no remarriage should be permitted (I think that's about how it stands now).