Hi Mr. Yuri, a SubD-friendly file is structured very differently than what you will have in an STL file. The STL file contains only triangles while a SubD file should have all or nearly all quads and be sparser.

It's not a matter of just packaging the data in a different file format, the actual structure of the polygons needs to be redone.

There is a process called "retopology" which can be used for that kind of process, usually it's a manual process where you sketch out a quad only structure over top of a denser triangle mesh. There are some tools like TopoGun that can be used for that and also ZBrush has an automatic process called ZRemesher that does that kind of thing as well.

- Michael