Force OpenSSH server to accept nonexistent useames?

Can sshd be configured to accept a nonexistent useame and log in as a predefined user, such as guest?

For example, if I enter ssh nxuser123@localhost, sshd should save "nxuser123" in an environment variable and log into the host as "guest."

I noticed a similar question from a few years ago, but it was never properly answered: Can I get OpenSSH's sshd to accept a login for a non-existent user?

I do not want to create the nonexistent user accounts as someone suggested in an answer to the above question.

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