For your specific case of “(X Mix)” appearing in the track title, you could try something like the following to add the value of “X” to the list of genres associated with the track:
$set(_working,$lower(%title%))
$set(_genre,$rsearch(%_working%,\\\(\(\\S+\)\\s+mix\\\)))
$if(%_genre%,$copymerge(genre,_genre))
Note that this requires that the track title contains “X Mix” enclosed in brackets, for the regular expression to match and extract the value of “X” to add to the %genres% multi-value variable provided from MusicBrainz (with the Picard “Use genres from MusicBrainz” setting enabled).