I tried dimensioning to an existing wall in a view set to New, a new door, and had no problems. Revit does not demolish an existing wall when you place an existing or new door, but does demolish the opening for the door to fit into.
I am not seeing what you are trying to do with the dimensions. Could you post an image showing the existing wall the door, Revit demolished opening for the door and dimensions?
Mel Persin, AIA
AEC Technology Consultant
Technology to Visualize and Realize Solutions
MasterGraphics, Inc.