Every field on the Tag Panel can have custom values added to its drop-down list. These can be customized via Options → Tag Panel and the Genre field is no exception.
You can use
Options → Tag Panel → Genre → Custom list values → Manage...
to edit the list of custom Genres and also hide the standard genres, if you prefer your own.
The following screenshot exemplifies the process:
Configuring Custom List Values on the Tag Panel can become tedious if the list of values to be added is quite long. It's also possible to import lists of genres (or composers, countries, ...) from text files. Exporting is also possible, so you can create a backup or share your lists.