Since you can’t stop a numeric field from having leading zeros when those positions are zero, I guess we have to assume that the question is about the result of converting a numeric value to a character value, either by assignment to a character field or by printing or displaying the value. The %editx() builtin functions can help for assignments. And edit codes or edit words can help for print or display.
An explanation of what is meant by “have a numeric field that keeps leading zeros” would be helpful.
Need your input – what are you trying to do and are you actually using RPG or RPGLE?