its possible but more as a ‘workaround’ than a defined function.
you can write the form instance number to a field, run a behaviour to check how LONG it is (1 char, 2 chars etc)using the ‘LENGTH’ function and then based on that result then use a ‘set field’ script to prefix the appropriate number of 0’s you want
There may be other behaviour functions
(http://www.perfectforms.com/Documentation/manual/html/functions.htm) that could do this more succinctly, but this is one way you could do it.
perhaps someone else has come across a better way and would like to share