I use Media Rage on my Mac to edit ID3 tags for my MP3 files. When you go to Preferences >Miscellaneous, you can enter in a list of genres next to Custom Genres that Media Rage will then use in dropdown menus and other places. Here's my list of custom genres, separated by commas as Media Rage requires.

Blues, Blues - Chicago, Blues - Country, Blues - Delta, Blues - Texas, Children, Classical, Classical - 20th Century (1910-1945), Classical - Baroque (1600-1750), Classical - Classicism (1750-1820), Classical - Early, Classical - Guitar, Classical - Modern (1945+), Classical - Opera, Classical - Romanticism (1820-1910), Comedy - Music, Comedy - Spoken, Country, Country - Alternative, Country - Bakersfield, Country - Bluegrass, Country - Folk, Country - Honky Tonk, Country - Neo-Traditionalist, Country - Old Timey, Country - Outlaw, Country - Pop, Country - Traditional, Country - Western Swing, Easy Listening, Easy Listening - Ballroom, Easy Listening - Lounge, Electronica, Electronica - Ambient, Electronica - Big Beat, Electronica - Dance, Electronica - House, Electronica - Jungle, Electronica - Trip-Hop, Gospel, Holiday - Christmas, Holiday - Halloween, Jazz, Jazz - Acid, Jazz - Bebop, Jazz - Big Band, Jazz - Classic, Jazz - Cool, Jazz - Crossover, Jazz - Dixieland, Jazz - Free, Jazz - Fusion, Jazz - Hard Bop, Jazz - Latin, Jazz - Post-Bop, Jazz - Ragtime, Jazz - Swing, Jazz - Vocal, Latin, Latin - Cuban, Latin - Cumbia, Latin - Dance, Latin - Mariachi, Latin - Pop, Latin - Reggaeton, Latin - Salsa, Latin - Samba, Latin - Tango, Latin - Tropicalia, March, Mashup, New Age, R&B, R&B - Chicago Soul, R&B - Dance, R&B - Disco, R&B - Doo Wop, R&B - Funk, R&B - Latin Funk, R&B - Memphis Soul, R&B - Motown, R&B - Neo-Soul, R&B - New Orleans, R&B - Philly Soul, R&B - Pop, R&B - Soul, R&B - Southern Soul, R&B - Urban, Radio, Rap, Rap - Alternative, Rap - East Coast, Rap - G-Funk, Rap - Gangsta, Rap - Geeksta, Rap - Hardcore, Rap - Latin, Rap - Old School, Rap - Pop, Rap - Southern, Rap - Underground, Reggae, Reggae - Dancehall, Reggae - Dub, Reggae - Ska, Rock, Rock & Roll, Rock - Alternative, Rock - Arena, Rock - Blue-Eyed Soul, Rock - Blues, Rock - British Blues, Rock - Britpop, Rock - Bubblegum, Rock - Celtic, Rock - Christian, Rock - Country, Rock - Dance, Rock - Dream Pop, Rock - Emo, Rock - Ethnic, Rock - Experimental, Rock - Folk, Rock - French Pop, Rock - Funk Metal, Rock - Garage, Rock - Girl Group, Rock - Glam, Rock - Gothic, Rock - Grunge, Rock - Hard, Rock - Heartland, Rock - Industrial, Rock - Jam, Rock - Jazz, Rock - Latin, Rock - Merseybeat, Rock - Metal, Rock - New Wave, Rock - Noise, Rock - Novelty, Rock - Pop, Rock - Power Pop, Rock - Progressive, Rock - Psychedelic, Rock - Psychobilly, Rock - Punk, Rock - Retro Swing, Rock - Roots, Rock - Shoegazing, Rock - Singer-Songwriter, Rock - Soft, Rock - Southern, Rock - Surf, Soundtrack - Film, Soundtrack - Musical, Spoken Word, Spoken Word - Audiobook, Spoken Word - Drama, Spoken Word - Poetry, Spoken Word - Speeches, World, World - Africa, World - Americas, World - Asia, World - Australia, World - British Isles, World - Caribbean, World - Eastern Europe, World - Flamenco, World - India, World - Latin America, World - Middle East, World - North America, World - Polynesia, World - Western Europe