I agree that you cannot hookup a GPS unit directly to Autocad to do what you want.
A little over a year ago, I purchased a program that allowed me to transfer Autocad 2004 Map drawings in NAD-83Iowa State Plane North and South Coordinate System to my Garmin GPS III Plus and Delorme Street Atlas Plus 2009. I can save as GPX files.
My project is drawing old RR lines over old aerial photos(1930s) here in Iowa. With these drawings in a GPS unit and Delorme on a netbook with a GPS attached, we can drive right to the old RR line, stop and look for evidence. No guessing where the old RR line was located.
I created numerous logs of my travels in Delorme and can convert the logs to a shape file in the program I purchased. Then import into Autocad 2004 Map to the Iowa State Plane Coordinate System. The shape file is a 3D polyline in Autocad and fits over the aerials just fine. Even had a log of a return train trip from Albuquerque to Galesburg, Illinois (over 80,000 points) that I brought into Autocad 2004 Map. It crosses a short segment of Iowa and fits on the aerial. Delorme logs record every second with the X, Y & Z.
So I am taking drawings from Autocad 2004 Map to Delorme and logs from Delorme to Autocad 2004 Map. I would assume that Microsoft Streets and Trips would be similar to Delorme. The program I purchased can import the MS Streets and Trips *.EST files.
Not exactly what you wanted to do, but there is a workaround. You will need Autocad Map to deal with coordinate systems.
atlast
Iowa