The Add to Existing option refers to what takes place when you first import raw data. So, for example if you had a topography composed of several adjacent map sheets you would select Add to Existing and then import each map sheet separately and put it into the one surface. However, it does not perform the function of updating a subset of the topography.
When doing this there are two distinct scenarios:
First is generally an easy fix. Use the following procedure: import the patch section as a new Inner Surface.
if there is a Data Feature within this surface which delineates the boundary, then open up a Data Window, right click on this feature and select Copy from the pop-up menu. Make the terrain window active and go edit paste and the feature will be pasted as a Surface Feature.
If there is no such feature then open up a rendered window for the patch surface and manually draw a Surface Feature as close as you reasonably can to the boundary of the surface. Then copy and paste this feature in a similar manner.
Create an Excavation Template, set the Gen Log to an unused Material Log (usually the default will do). Make sure Excavate Only and Fill Only are both deselected, bind the template to the patch surface.In the Terrain Window draw an Inner Polygon following the feature line which delineates the patch surface. Apply a nominal batter angle and excavate. The topography will be modified to reflect the patch surface.
If the patch section is beyond the bounds of the existing surface then you need to replace the existing topography. This will replace any existing reshaping with the original topography. Before starting this use Terrain/Copy to Inner Surface to copy the current terrain to a new Inner Surface. Load the new topography then start by using the above process to impose the current surface (one that we just copied to a Inner Surface) onto the updated terrain.