Then you will add these three pieces to your Existing Assembly (do this to both sides)
Three Generic Links, one vertical at pavement depth or all the way to base course.
Then a link width and slope, don't worry about width or slope right now
Then a Link Slope to surface
The key is in the assembly properties you will tell the first horizontal link to get it information from bottom link of your pavement section, and the outside link to surface will get it's slope from either the link before it or the same pavement/ subgrade link.
I left your section out for clarity.
Now build your design corridor with this assembly, and trget the surface from the helper corridor.
When you are done using him set his display properties to invisible.