You can probably import .pns file to Pegasus Mail and use the signatures for new email messages, which would be auto-saved as .asv files. You cannot directly convert .pns to .asv as this conversion would not make much sense.
Convert Pegasus Mail formatted signature file to Pegasus Mail auto saved message file.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert pns to asv on Windows
One of the oldest e-mail clients available
Convert from pns 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.