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

[MRob]

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

set "${env.vnd.dovecot.username}" "someu...@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?