If you read back a few years, you'll find reports that kludging the user into the database can and will cause issues with updating. Register with a throwaway email address or simply make up an email address (somename@erewhon.net for instance) since Kobo does not check to see if you used a real email address.
See
Bypassing registration in 3.19+ for an example of someone who had issues with a firmware upgrade.
Personally, since bypassing registration is not a supported action, you are taking your chances for no real gain.