SFZ offers a lot more possibilities than soundfonts and the construction can be very complex, so it can happen SFZ-files contain opcodes which are not supported in the soundfont format. In that matter it's impossible for Polyphone to read all SFZ-files correctly. As Polyphone is now I think it does a very good job with reading SFZ-files.
The best you can do is to get a bit familiar with the SFZ-format. Then you can alter the SFZ-files yourself, so it can be readen with Polyphone. To alter SFZ-files you can do a lot with just a simple text-editor and its "search and replace" function.
To learn the basics of the SFZ-format you can do the opposite: use Polyphone to export soundfonts to SFZ. Then open the SFZ-files with a text editor to examine its contents.
More info about the SFZ-format:
sfzformat.com/
sfzformat.com/legacy/
www.drealm.info/sfz/plj-sfz.xhtml