How can I create a final surface from the existing and the proposed surface?
If you wish to merge your design surface with your existing surface, you should use the command Surface > Edit Surface > Merge Surfaces.
This command combines the data contained in one surface (the Original surface) with the data in another surface (the Design surface). The software places the combined data from these two surfaces into a third surface, the Destination surface. The two surfaces you want to merge may overlap or be adjacent to one another. In areas of common coverage between the two surfaces, the data from the Design surface, rather than from the Original surface, will be placed into the Destination surface. However, you can choose to also include points from the Original surface in areas of common coverage by using the Retain All Original Surface Points option.