Let's start with a simple Revit toposurface.
Export to Max by using Suite Workflows - 3ds Max Design Exterior Rendering
Once in Max, export selected (the toposurface) to OBJ using the Mudbox preset.
Open meshmixer (now at version 08). File - Import the OBJ. Start playing around with it, using the brush tools to smooth or warp the site. You may need to 'scale' up your active tool by using the middle mouse button to see real results.
I added some body parts to the site:
Export to OBJ from meshmixer.
Back in Max, select the previous surface object. Then Import, select the new OBJ you just made.
Now, export selected the new OBJ (with morphs) to a DWG file.
Import the DWG in Revit. If you haven't gone too wild, you should be able to use Auto - Center to Center option effectively.
Finally, Revit won't like this import very much - it probably won't find points to make a surface. You will need to round-trip this DWG import by exporting out to DWG from Revit and then importing it back in again.
The 'round tripped' import symbol can be used to generate the Toposurface, now morphed:
You may also be interested in:
Freeform and organic modelling from MeshMixer to 3ds Max to Revit