I think you can also say.

"Soy del Liverpool FC" or Soy del FC Barcelona.

But this is only for sports.

Can someone confirm this?

Yes, only for sports and only in a clear context where it's obvious you don't mean to say you play for those teams. :D

Also, only for sports and only in Mexico, you may say: "irle a (un equipo/jugador)"

- Les voy a los Medias Rojas.
- Beto no es mi amigo porque le va al Real Madrid.
- ¿A quién le vas?
- Le voy al corredor de la izquierda.