you can use something like:
=MID(B3,10,1) & MID(B3,9,1) & MID(B3,8,1) & MID(B3,7,1) & MID(B3,6,1) & MID(B3,5,1) & MID(B3,4,1) & MID(B3,3,1) & MID(B3,2,1) & MID(B3,1,1)
if you want to reverse the string in B3 AND YOUR STRING IS NO LONGER THAN 10 CHARACTERS...
this is not very elegant, but you dont need a macro or udf.
if you want to apply this to larger than 10 characters, just reply the MID(B3, XXX, 1) module in the beggining as long as you want.