We have found one software suitable in our database for this conversion.
Keynote (previously part of iWorks) supports presentation formats from Microsoft PowerPoint, so you can simply import your .pptx files to Keynote and save it to one of its default file types, such as .key files.
There doesn't seem any 3rd party software that could save in Keynote presentation format, so using the original program most likely remains your only viable option for pptx to key conversion.
PPTX
Files with pptx extension are presentations from modern version of PowerPoint from Microsoft Office / Office 365. PowerPoint slide shows are basically set of screens which can include text, graphic, sounds or videos used for various presentation purposes.
KEY
Files with key extension can be often found as presentations saved in latest versions of Apple (iWork) Keynote, a professional presentation tool for Mac OS X, macOS and iOS.