In reply to Si + Kim: Don't do it... But if you must (and you will not have a good time, especially if you snap a tib or fib), look for old Silvretta bindings - the 404 would be favourite. These have a step in similar to a Crampomatic Crampon, so will fit any B3 with a heel and toe welt, but offer some release at the heel.
A B3 boot does not offer the rigidity that is necessary to protect the ankle and lower leg, so be VERY careful, if you do follow this route. I have heard people talking about using boots such as Spantiks for skiing - THIS IS A CLIMBING BOOT - don't be so stupid
(By the way - I have spantiks - I use them for Himalayan climbing...)