Ability to generate a different username than User Name Generation Rule setup in security console

Summary:

Hi All, 

Currently we have our "User Name Generation Rule" setup to "Person or party number" based on which user account is created for all pending workers with person number as the username.

We wanted to check if there is possibility of keeping the User Name Generation Rule to "Person or party number" but have the flexibility to create the user account with a different user name like Firstname.Lastname or personal email id defined on the pending worker.

Please let us know if this is something that can configures in the system and if yes, then please guide us through the process.