Custom Naming Format for User Creation Template

Custom Naming Format for User Creation Template

I'm trying to create a custom naming format so that the logon name is the first letter of the first name and the first letter of the last name plus 001, i.e. Joe Bloggs = jb001

The 001 would then increment if we had a user with the duplicate initials, John Brown = jb002.

I've managed to set up the naming with a format value of %givenName%1[L]%sn%1[L], but just need to add the incremental number.  

Is this possible?

Thanks.

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