It's not thaaat easy, but it is possible. First of all, you have to convert the numerical attribute into a nominal attribute with Numerical to Polynominal. Then you have to create several regular expressions. For 3-digit values in the form:
^(\d)$ -> 00$1
^(\d\d)$ -> 0$1
The caret specifies that the expression must be matched at the beginning of the string, the dollar stands for the end of the string. Both together require an exact match.