Here's a few off the top of my head to start us off..

Matt Skelton
Elvis Parsley
Pele Reid
Troy Dorsey

Both Nigel Benn & Ricky Hatton dabbled in amateur Kickboxing for a while, but I believe that was before they became amateur boxers..

Dorsey however, was a world champ in both Boxing and Karate..

Is He the only one?