There are 2 ways:
a) You can modify the used Websource Script manually* and add the fixed GENRE to the code
or
b) Load all songs into Mp3tag (for example once a day), select the ones that should become the same GENRE, type your content in the Tag Panel in the field GENRE and save it to all your selected tracks.
* For Discogs you would have to modify the Discogs.inc text file at line #240, #241 and a new #242 like this:
this would result in:
Of course you can choose whatever GENRE content you want instead of "fixed pgh-Genre".
As this Websource script is an official one, it will be overwritten with the next Mp3tag update.