Moodle allows you to add new profile fields for users and external database authentication supports importing values to these fields. The new user profile fields appear at the end of the list of fields under the Data mapping section on the external database authentication plugin configuration page.
Modifying the Moodle database
tables is a bad idea, it can cause problems with upgrades and it means
when providing support (e.g. on the forums) your Moodle instance is different to a normal version.