It appears that fm3 to wku conversion is related to export of old Lotus 1-2-3 spreadsheets, perhaps for users that still use this software on some of their machines for a particular reason. If you need to, you can download old versions of Lotus 1-2-3 from abandonware websites, but perhaps it's better to keep data in some generic exchange format, like CSV.
Updated: October 3, 2023