I replied in the other post too but there’s nothing that advanced for retopology in the works for Rhino SubDs. You can of course import FBX or OBJ from Blender and run ToSubD, so there’s a workflow you could try now. Use the Subdivision surface modifier in Blender to get a good idea of what the Rhino SubD will look like. Once in Rhino WIP, you can also use Pull to pull the SubD edit or control points to target geometry too.
In the Rhino WIP itself, you can use the 3DFace command to make your first face and then Append to build off of that while using vertex Osnaps to an underlying mesh. An alternate workflow would be to use the QuadRemesh command to make a quad mesh/SubD over the original mesh. This is harder to control topology wise but you can indicate guide curves in the QuadRemesh command to help.