Hi,
The file extension doesn't really mean anything. By default Windows associate .ins to Internet connection but it's just an extension. If you rename a .mp3 to .docx, it will not become a Winword file

but you will see the Winword icon in your file system.
So, if your .ins contains ASCII, and it does well formatted for instrument definition, you should be able to import it
from SONAR in the appropriate dialog box Preferences > MIDI > Instruments > Definition..., no matter of the file icon.