If you want to give your Pleroma users an XMPP (chat) account, you can configure MongooseIM to use your Pleroma server for user authentication, automatically giving every local user an XMPP account.
In general, you just have to follow the configuration described at https://mongooseim.readthedocs.io/en/latest/authentication-backends/HTTP-authentication-module/ and do these changes to your mongooseim.cfg.
{auth_method, http}
.{http, global, auth, [{workers, 50}], [{server, "https://yourpleromainstance.com"}]}
Restart your MongooseIM server, your users should now be able to connect with their Pleroma credentials.