You might find a better way, but how about:

= REPLACE(STR(15, 5, 0), ' ', '0')

STR(num, length, decimals) function converts the number to string of given length (padded with spaces) and given number of decimals.

Alternative?

= RIGHT('00000' + @number, 5)

Remember you should handle the formatting in presentation layer, but sometimes you might find this useful…