Creating WordPress Users With Username “firstlast” Instead of Email?

I use FluentForms Pro to add new users to a site. They are currently set with usernames = email. However, that prevents them from changing their email address. If they do and want to sign in with username/emailaddress. The username is still the old email address causing a lot of confusion. I’d rather make the user name firstnamelastname (.e.g, John Doe becomes username johndoe). However, I cannot prefill the username like that. I can use any field as the username but not a combination of two. So I can use the first name or the last name, but not both. The username in the integration does not accept merge fields either. Can this be changed, please?