The way I read the rules this seems to be allowed, yes.
Though it allows for a fresh, newly-created druid or sorcerer to do this:
- Eschew your initial general talent for Rank 2 on your magical path.
- Chance-cast your favorite Rank 3 spell and pray to Rust you don't die in the process.
- Grab some parchment and ink, succeed in your Lore test, and write it down.
- Go wild, happily casting the aforementioned Rank 3 spell from your grimoire like a boss - not bad for someone who has just learned to cast spells, eh?