How can I convert a wall to a writing board?
whiteboard paint, quick google search should give you a few options for that:
Google Search

another idea is vinyl wrapping the whole wall, but that could cost more than the whiteboard paint - The wife wraps boats in vinyl, and says it would cost about 200 quid to wrap a 7m2 wall and all you would need to do is give it a really good sand.

She also has in the office a white board they made with the vinyl and it's about 5 years old and still going strong :) 

Hope that helps.

Martyn