Beside this ... Actions
... there is also the possibility to create a multi value GENRE tag-field, using the action "Format value" on the tag-field GENRE with a Formatstring containig values, which are separated by the "Double Slash" separator, e. g. 'Rock\\Trash'.
Use the dialog "Extended Tag View..." to display all tag-fields with their content.
Action: Format Value
Formatstring: $if(%genre%,%genre%', ',)new genre
Comments: ', '
this is the seperator between the old and the new genre. You can replace it by '; ' or '/' or whatever you like. Thanks to the $if function, it is only written when there is already a genre.
you may have guessed it, you should replace this by the name of your new genre
If you are multivalue tags for GENRE (displayed as genre 1\\genre2\\genre 3 in Mp3tag), you have to use this:
Formatstring: $if(%genre%,$meta_sep(genre,\\)'\\',)\\new genre