Is it possible to create a custom user name generation rule?

Summary:

We have a requirement to create a username with prefix 'e' followed by person number. But seeded functionality didn't deliver the generation rule that suits our requirement. Is there a way that we can add custom generation methods?

Content (required):

The actual problem is that as soon as a pending worker is created a default username is being generated along with On-Boarding checklist being assigned to the employees. Later the username is being updated by REST APIs . The checklist notifications that were generated earlier are now broken due to change in the username.

Version (include the version you are using, if applicable):