iWorkks Keynote supports presentation formats from Microsoft PowerPoint, so you can simply import the PPTX file to Keynote and save it to one of its default file types. There doesn't seem any 3rd party software that could save in Keynote presentation format.
Convert Microsoft PowerPoint Open XML presentation to iWork Keynote '09 presentation.
We found one software available in the database for this conversion.
Apple macOS / Mac OS X software - convert pptx to key on OS X
The most popular presentation software for Mac
Convert from pptx file format
|Yes, Keynote for Mac supports pptx file conversion as a source file type.||Yes||Yes||pptx editor||Yes||No||No||No|
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.