Character Controller walking on walls?

Working on a game where the player will have the ability to walk on specific walls/ceilings. Unfortunately, I'm already pretty far in programming my game mechanics and I've been using the default Unity 5 FPS Character Controller so far, and I've heard that the default character controllers basically make walking on walls virtually impossible. Would there be a way to alter the gravity vector in the Character Motor script to make walking on walls possible? Or would I have to do that in addition to programming a unique script for walkable walls that alters player direction upon collision? Any help would be greatly appreciated. Thanks.