First, formats other than OU, UU, RU, NU, PU, and ZU are allowed to completely ignore tiers and ban whatever they want. Monotype, for example, allows speed Deoxys (an Ubers Pokemon) but bans 10% Zygarde (a RU Pokemon).
Can I use lower tier Pokemon in higher tiers?
As alluded to in the previous answer, yes, of course! You may not use a Pokemon in a tier lower than the one it is officially listed in, but you can certainly use a Pokemon in a tier higher than the one it is listed in. For posterity, the order of tiers from highest to lowest is: Ubers, Overused (OU), Underused (UU), Rarelyused (RU), Neverused (NU), and PU.