Is possible to override Dovecot variables or Sieve env vars and extdata lookups

Are the dovecot environment variables mutable? I'd like to override like:

set "${env.vnd.dovecot.username}" "someuser@example.org"

Or better, I prefer to override %u Dovecot variable during the userdb lookup.

Is either possible?

PS, when Sieve does extdata lookups for username key, where does username come from? %u? Does an override as above change the dict lookup key for extdata?