Enter my colleague Mitchell Parsonage, who managed to create a workflow with three easy steps :
1. Define your site extent in Infraworks and export to an OBJ format
2. Import the OBJ into Autodesk Formit, and Export to DXF
3. Link the DXF into Revit and generate a Toposurface from the Import
4. The end result? Nicely done Mitch!