I'm pretty sure Cosmic Prophet made staircase like that, you can find it in his free Dark City pack.
You can find this pack here https://starmind3d.weebly.com/model-packs.html and also some other useful stuff
If you already have staircase model but can't walk on it, check it's collision in fpe file, Set it to 1 (it's per polygon collision), it might help, although FpsC physics sometimes is more tricky than it should and just might won't let you climb up your staircase
But hol-up, there is a little trick
You can always place invisible wall right over stairs (place it carefully so player wouldn't notice), now engine would detect you climbing over flat surface - its waay easier for FpsC to calculate, and less buggy