Making a path less slippery
Anything you add on top will likely wash away. So I agree with the rest that steps would be the best solution. 

An easy way to add steps may be put a board on edge the width of the path every 3-5 ft. Then fill in behind it with stones or gravel. The boards, be sure to use pressure treated, would keep the gravel from washing away. The gravel would give you flat sections to walk on which would reduce the slip issue.

I think this would be much easier then building an entire wooden or stone staircase.