It wouldnt technically be hard - just a butt pain.
You would need to have an option for which signature they were writing when editing their sig. How many they can have, what if they can have 3 but only have 1 filled out? How are you going to store them (user profile fields?) how are you going to pull them (random during constructing the post?).
Its a nice idea - and not hard to do - but a lot of work to make it decent.
Good luck