Is it possible to combine 2 separate CSV files from the same job (continuing areas) via the import function?
You can definitely add more than 1 survey to the design.
So you can go back to site as well and just add more survey points if you have the exact same origin point of reference.
When you do just make sure that you 'untick' the clear all existing feature points box. I circled it in red. If you don't do this it will clear the first set of points.
Also just looking at the data you can make use of the interpolate terrain points feature where you see gaps in the terrain.
To do this, just click on one of the ground points, and to the right hand menu, click where it says interpolate terrain points and then just click on the 2nd point (where you want to fill in a gap).
This can be very useful for you especially if the terrain is relatively flat.
Also all interpolated points are named as "interpolated" so you can see which ones you have data for and which ones you're telling the software to assume for the purposes of design.