I'm not sure why you would want to do this, because that's now how exp works:
the + is a raw value, whereas your exp is a percentage e.g.
If your exp is 54.678%, +20, is not going to make your exp 54.698%.
Anyway I took the time to find where the sprintf format string is; Search referenced strings in OllyDbg for "+%I64u"
Example:
Search - Text strings referenced in Conquer, item 2283
Address = 00467B05
Command = PUSH OFFSET 00504B58
Comments = ASCII "+%I64u"
So bottom line, what your trying to do won't work and doesn't actually make any sense, because it's formatting a 64bit unsigned integer (not a percentage / floating point).