Wizards, Magus's, Sorcerors, and Bards can add spells to the spell lists from other sources even if its not on their normal list or is a divine only spell. Is it possible for Druids, Rangers, Paladins, and Clerics to do the same then?
I say you should make it so the tree is only willing to listen to the King or Current Ruler and make it Protect some area so that the party can't travel with it and what ever 'Hex' it is in have a lower random chance counter for Monsters.