But here's what I was thinking.
the Fighting-Man in question would go to Level 3 plus 5,000 XP. If he lives that long and his INT is 9 or more, and he knows a friendly Wizard who has Knock, he could spend 1d6 weeks and 5,000 XP plus however much gold the Wizard might like, and he can learn it.
He wouldn't know how to read any magic or write a scroll or anything. He could just cast Knock once per day. I'm not sure how you would handle a spell book for such a fellow.
What do you think? Too strict? Too lenient? Not allowed at all? Tell me!