Both .skd and .skf formats come from old software called AutoSketch, which was a 2D vector software marketed by Autodesk decades ago. Modern software usually do not support these files and since they cannot import / read these files it goes without saying that no conversion is possible as well.
The only way how to work with these files is to obtain the original software, for example you can download various versions of the software in the WinWorld library. If you manage to export your ancient AutoSketch files to modern software, it might be possible to export them further.
Updated: October 12, 2023