Fighters who want to convert to a life of a magic can become Geomancers. Once they choose to change, they can still use the armor, weapons, and magic items that the fighter classes use. Beware: Bards lose their songs, Warriors lose their multiple attacks, Hunters lose their critical hit ability, and Monks lose their armor class bonus and multiple attacks. It's part of the trade off. To become a Geomancer, you must find the one special location that performs the rite of passage to this mage class.
For a list of Geomancer spells and their effects, visit BT3 Spellbooks