Can we make username=sub?

I am using ForceAliasCreation and only required attribute is email for signup. User can sign in through preferred_username and email. But During sign-up it is asking a username that is not preferred_username & that is immutable. After email conformation user can chose their preferred_username. Now this is very odd that one user have 2 username one is immutable and other is mutable. Preferred username makes sense because user can change it later on.So I don't want to tell user that username required during signup instead I will make it equals to sub .Is it possible?