Quickly adding several genres to a single genre tag's field

My intention is to rapidly input comma-delimitered genres in the single genre tag space without having to repeatedly type the same words. The mp3tag program offers a dropdown yet it's unsurprisingly of little use when anything that's not a "\"'d is a single genre/word. Nevertheless, I understand that multiple tags are not universally supported by all softwares and formats and comma separation works for my needs.

The goal is to come-up with an easier way to quickly input genres without having to retype words. Although I could - as an idea - utilise text expansion (eg. Better Touch Tool) to convert shorthand into words and commas, I do wish to know if mp3tag can offer workflows I'm unaware of. For example, I believe there may be a way to script multiple tags (after utilising "\") back into a single comma-delimetered field when ready to save edits; that way, the genres dropdown and custom genres section aren't a useless mess. It's not the best idea, and neither am I well-versed in mp3tag scripting, yet I hope my goal is clearer.

P.S Using mp3tag Mac

On the basic problem: I would assume that there is a list of typical combinations. Or a set of typical genres that you listen to and which may end up in such a list.

You could create a set of action groups that adds a single genre to the already existing list:
Action of the type "Format value" for GENRE
Format string: %genre%\\Blues
(where "Blues" is just an example genre.
You then add another action to the action group of the type "Merge duplicate fields" for GENRE with the separator that you want to see.
If you then execute the next similar action group with the
Format String: %genre%\\Rock
Then this will first create another field of the type "Genre", this time with the contents "Rock" and keep the previous field.
The following action to merge the fields and with a comma as separator will then lead to
Genre: Blues,Rock
and so on.