Your class spell list is your class spell list. I guess you got hung up on "regardless of the source," which was probably meant to cover roleplaying situations, i.e. things that aren't scrolls or spellbooks. You can never add to your spells known for one class something that is not in your class spell list.

If you want to learn the spell anyway, there are provisions for learning "unusual spells" in PHB/SRD, under the sorcerer description. You could talk to your DM about doing research on that particular spell to eventually learn it. Personally, if I allowed it in one of my games, there would be lots of challenges/quests involved, since you're already a mighty wizard and you don't necessarily need to be stepping on the mighty cleric's mighty toes.