It is unlikely you could convert .ckt files to .ptn files in anything else but CADKEY CAD 3D modeling software. This conversion is basically exporting of CADKEY templates (.ckt) to a finished CADKEY pattern file (.ptn.). Probably done through default File ► Save as function.
Convert CADKEY template file to CADKEY pattern.
We found one software available in the database for this conversion.
Microsoft Windows software - convert ckt to ptn on Windows
A program used for modeling CAD 3D wireframes
Convert from ckt 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.