Ok, this should do the work.

=(CODE(A2)<58)&A2&REPT(0,50-LEN(A2))
I appreciate your help, but this does not work either. When you use the above to sort:
AAAB
99
AAA1A
346AA
ZAAA
AAAA
5689
4AAA

you should get:
AAAA
AAAB
AAA1A
ZAAA
346AA
4AAA
5689
99

Is that what you get? (It didn't work for me)