I don't think there are expressions that will enable a character to do a whole walk cycle.
However, there are melscripts and plugins out there that do the "footstep" method of animation.
(i.e you place a "footstep" locator where you want your character to "walk" to, and the character "walks" to/on it/them.)
Israel "Izzy" Long
Motion and Title Design for Broadcast-Film-DS
izzylong.com