inroads existing pavement box in cross sections

I'm trying to figure out how to display my existing pavement layers in my proposed cross sections.  My existing surface was created by survey and I roughly have 3 points for the existing pavement (left edge, cl, right edge).  I know the pavement thicknesses but have no idea how to make pavement boxes using existing data.  I've made them for proposed roadways using the roadway designer with the proposed horizontal and vertical alignment but I'm not sure how to do it for existing.

I've seen videos for geopak but not for inroads.  From what I'm guessing, you need to create an alignment using the existing cl feature.  You can then use this for your vertical as well.  You then apply a template to it but use point controls with both vertical and horizontal controls setting the pavement box edges to the edge of pavement features.

Please let me know if there is an easier way as this seems way more complicated than it should be.

After I'm done is there a way to combine with the old existing surface without doing a merge?

Thanks!