CKD files are special format of CAD designs files unique to CADKEY, the predecessor of Kubotek KeyCreator and not many tools can actually still open or save to that format.
While PRT files are used in many software, a great deal of them was actually made using CADKEY as well, so prt to ckd conversion is simply export between two different types of CADKEY formats.
As such, your best bet is to use the KeyCreator, that is unless you have access to the original CADKEY. It is unlikely any other software can help you with this otherwise, so if you face some particular issue or problem with such export, it would be best to contact Kubotek support or reach out to the KeyCreator user community.
Updated: August 31, 2023