Is it possible to add more admin groups to the oxide.config.json file like in the example below?

"Options": {
    "Modded": true,
    "PluginWatchers": true,
    "DefaultGroups": {
      "Players": "default",
      "Administrators": "owner",
      "Administrators": "head"
    },

Not at the moment, but you could always write a plugin to do that or use an existing.

Ok thanks