This is sort of tough to do in hammer. If you use the arch+add height+vertex edit method you might end up with a bumpy uneven ramp that’s ugly as hell. There’s a tutorial for that here: http://www.interlopers.net/index.php?page=tutorials&id=3503
I find that it’s better to convert the top faces of your stepped arch into displacements. After that you manually raise each displacement vertice using “raise to” mode, adding on height to each vertice until the whole thing looks like a ramp. It takes a little bit of time and math, but unless you can make ramp model in a modelling program this is the smoothest curved ramp you’re going to get.