Whether there is a possibility to import users into lists of recipients/senders via web-gui? Or will such functionality appear?
Postfix has an API. Use can use that for bulk imports.
If your postfix users are in a Radius or LDAP directory, you can use those as authentication sources in OPNsense.
You should get familiar with the API to do this:https://forum.opnsense.org/index.php?topic=6116.msg25681#msg25681
curl -X POST -d '{"recipient":{"enabled":"1","address":"<email>","action":"OK"}}' -H "Content-Type: application/json" -k -u "<key>":"<secret>" http://<ipaddr>/api/postfix/recipient/addRecipient