Additional stencils (smartshapes) may be imported to Visio and used as symbols for any Visio drawing created with the program. Microsoft offers some huge packs with shapes for free on their website.
Convert Microsoft Visio smartshapes data to Microsoft Visio drawing.
We found one software available in the database for this conversion.
Microsoft Windows software - convert vss to vsdx on Windows
A vector based diagramming software from Microsoft
Convert from vss file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.