Both .xfdf and .fdf files are forms used by Adobe Acrobat. If any program can somehow transfer data from one format to another it would be Acrobat. Unfortunately, we couldn't find any other relevant information that would users with xfdf to fdf conversion.
Convert Adobe Acrobat forms document to Adobe Acrobat Portable Document Input Form.
We found one software available in the database for this conversion.
Apple macOS / Mac OS X software - convert xfdf to fdf on OS X
Mac version of Adobe's PDF handling software
Convert from xfdf 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.