No easy workaround that I can think of if you want to increment the user id based on some fixed pattern independent of the user - e.g. starting with user000001, and then incrementing up to user999999.
It would be a lot easier if you were to generate random user ids instead. That way, you could use the "uniqueness checker" to see whether the randomly generated id had been used before.
You could even take the same approach if you were to have a fixed pattern id which just got incremented in the case of collisions. E.g. user id is first initial and last name and incremented for duplicates. So user John Doe or Jane Doe would get id jdoe, jdoe1, jdoe2 etc. If the uniqueness check on the base id doesn't work, just keep incrementing until you get a positive result from the uniqueness checker. Assuming your base pattern doesn't result in too many collisions, there won't be many loops in your PX.