It is unlikely that there can be a one-to-one correspondence between Tuleap users and Bugzilla users. Neither require emails to be unique per user and bugzilla has the user ID as optional. https://wiki.mozilla.org/Bugzilla:REST_API:Objects#User

Therefore, an option could be to create a generic bugzilla user in tuleap and use whatever user details are available from bugzilla as a comments in the changesets.