I successfully migrated users from V8 to V11 in this scenarios -- specifically users that were added after starting the upgrade process.
After the upgrade process there were about 30 new users that were added during the transition window. We were able to use Kentico's Export/Import capabilities to transfer the users, one user at a time. We exported each user from the Users module, and then imported them in the Sites module. We were very careful to make sure only the user object was imported.
To my surprise, this worked. Even the passwords were still intact. We only had trouble with a small number of users, because their passwords did not satisfy the upgraded password rules. For those users, we provided instructions for them to reset their password.
This might work for you too. I'd test it carefully, before trusting it.