Unfortunately making a ramp from a foundation or wall for that matter and having it in on the ground doesn't appear to be possible. That would entail a floor from one level to another. The game currently only allows stairs to bridge levels like that. I've wanted one for my toddlers to move from one level to another since the parents left them alone on one floor and for a story I had in the works a long time ago involving a sim in a wheelchair.
Using cfe I've never been able to get a satisfactory spacing between the two elevations that I could live with. Not to mention not having the actual connection between them for the toddler's movements. But using by using stairs to connect the ground and higher elevation, then using cfe to basically level the stairs (raising the terrain at the lower end) I was able to build the sloped ground to create the ramp.
Hope this gives you some ideas. If you do find a way to connect the two elevations with a floor, please share
