>...you would have a key of 0123400456 if you lead padded or 1234045600 trailing

The leading zero version would work, but you wouldn't want the trailing zero version, because in your example above you could not distinguish between 1234 and 12340. They both would be represented as 12340.

Anyway, I think Andy got good suggestions to use BINTOC(f1)+BINTOC(f2).