I would like to be able to select objects from within an xreference and utilize them for creating a surface, For example when you have the existing survey information xreffed into you design, and the existing surface doesn't have all the data that you need within it. In our organization we are not supposed to edit the survey, and correcting the surface has to be from within the existing survey. So what this requires is convincing the Survey Division that you need the information added to the surface, and then having them update their surface.
Activate the Terrain Model Explorer, select new surface, apply edit history, select contours or points, or..., when prompted select objects within the xref and when the selections have all been made build your surface.
Terrain Model Explorer