The standard sees one value per tag field.
Using a comma separated list of genres does not comply with the standard - even though your player may show this list.
Other players use "semicolon blank" as separator between values but still use only one tag field for e.g. GENRE.
Other players understand multivalue fields which are several fields of the same type.
So I doubt that a way to create a non-standard tag field content should be implemented.
You could also think of using something like your own fixed list of genres
1] You decide to use for example POP, POP ROCK or ROCK-POP, ROCK, ROCK-ALTERNATIVE, HARD ROCK or ROCK - HARD etc.
2] You add a menu of actions under the Actions Groups icon, that would put one oft those pre-chosen by you names in the GENRE tag field. You can also put more than one in the tag. You can also write these names by hand, not using the action. After recent change
it is possible to use a multilevel menu
3] You create an action to sort them out in an alphabetical way:
4] You can also use some other tag field than the GENRE [even a made up one] but being aware of that some players may not display some date properly or even whole tag fields
[Please note that some of the post in the threads that I evoke here are messed up in display - they should be taken care for as I already flagged them for attention by the stuff - so if you want to implement solutions discussed there please wait some time]