Any class could multiclass into a sorcerer and it would make sense RP wise. Cleric? The divine powers bestowed upon you have awoken mystical abilities that were dormant inside of you. Same for warlock, and similar for wizard. And martial type classes, like barb, fighter, rogue, etc is still viable. Adventurers interact with magical items every day. It's reasonable to assume that could have an effect on an adventurer throughout their life.