If your "Surface" is a TIN only (i.e. it has no linework, boundaries or points) you have to first create some linework - easiest way is to export to DXF 3D Faces and then reimport it and then you can add the 3D Faces to the Original Ground Surface model using either the Categorize method or simply add / remove surface members to the Original ground surface created for Takeoff. The connection to the added data is maintained and each time you rebuild surface those additional data items are added. If the Surface that you start with is made up of Points, Breaklines and Boundaries, then you can create your Takeoff Surfaces (you need at least one layer assigned to Original Ground in Categorize takeoff Layers) even if that layer is empty in order to Buiold the Takeoff Surfaces. Once created you can then use Add / Remove Surface Members abd either add the Surface (which by default adds all the points, breaklines and boundaries) to the Takeoff Surface. However at this stage we cannot add a TIN only - the way that would have to be done is to create 3D Faces from the TIN as Breaklines and then add them to the Takeoff Surface (which of course would be possible just isnt implemented in the current version.
When you imported the TIN only model in the first place, if you use the Import command as opposed to rag and drop, you can explode it during Import to Poly 3D to create the 3D Faces. Those faces can then be added to the Takeoff Surface manually using Add / Remove surface Members or the layer that the faces are on can be categorized to Original Ground and then they will get added to the surface model at the next Rebuild of Takeoff Surfaces. (This is what I would do with CAD TIN Models or LandXML TIN Models as the source data).
Note the Add Remove Surface Members can also be used to add Point Clouds *that have no layer" to a Takeoff Surface.
Hope that this is helpful
Alan