No, this is not a practical application. While a Terrain Model is a native Element and the equivalent of a DTM, it has many differences.
In Ss2, the DTM contains 3D Features and components. I it loses all of this when converted and simply represents the triangles. However, it is possible to have the 3D linestrings representing breaklines in a file and when these are edited, the Terrain Model will update to reflect the edits. The problem is, the tools for editing the 3D linestrings are not the tools you may have used in Ss2. During the import, there is an option to import the breaklines as 3D elements.
If you want to convert a design - first, you can actually import an IRD. However, there is a dependency issue that can cause the import to fail. You would need to import all alg alignments into Civil Geometry. Next, if you had other non-geometry controls, such as other corridors or Breaklines, extra steps are needed before the import will work.
A big thing about Ss4, is that you generally work in a 2D model and when 3D is needed, a 3D model is created in your file automatically and attached to your file. Then, as you create models, their geometry is modeled in 2D and they are given profiles to define the elevations along the geometry and Open Roads creates the 3D breaklins to represent you model in the 3D model. The only time you should work in a 3D file/model is when bringing in Survey Data.
Your templates can be used, but if you need to also use them in Ss2, you should make copies in case it adds things that Ss2 does not recognize.