How do you combine volumes/polygons from steps into one ramp? If I had a volume for each step, I'd have to do A* through each step.

Also, how do you do A* through the volumes/polygons? Do you have entrances and exits between each volume? How would you cut a shortcut across several volumes if there's a direct path?

This link has a full explanation of how Navmeshes are created and used:

http://www.critterai.org/nmgen_study

Edit:

Read your question wrong, sorry. The link should help you.

Simple answer - Nav meshes are usually a seperate mesh simplification of the actual scenery geometries.

The stairs would often be simplified into a flat ramp surface

I still don't see how navmeshes are used. Need more info.

I'm sure you can find something to read rather than having us copy and paste it here. There's no shortage of info on navmeshes on the web.