there is a super easy way to do it, since it is in first person.
you make sure your character's physics is set to "character". When that is done, create the stairs (not too steep) and put a plane above them which is tilting in line with the edge of each step. This will make sure your character can get up the stairs.
However, this is only an easy way of doing it (and most likely the worst way in terms of bugs and such). But it works.