In order to do that, you would have to modify the code of pointshop deeper. This poinshop (_Undefined's, I assume) is made to work with just the default Garry's Mod groups: superadmin, admin, and user. This is a recurring theme in addons, and you will have to modify each one if you want to allow only an "owner" group.
For this reason, many people here on the forum suggest that you use superadmin as it was intended: as the highest rank. Get a simple chat tags plugin (there is at least one that I know of in the 'Releases' section) and change your tag to "Owner" while still being in the superadmin group. You can read more on the whole owner group thing here:
http://forums.ulyssesmod.net/index.php/topic,6291.0.html.
