Comment 2 for bug 663584

Exporting to FXG is now available in the trunk (see Bug #625140 “Support export to FXG”).
It doesn't convert all the SVG features, but some just don't exist in FXG and thus are very difficult to implement with workarounds. I'm still working on it anyway. Details available in the report.

Not sure the opposite conversion is worth the effort. As far as I understand, most Flex users just want to use Inkscape as their vector editor, and importing FXG probably is far less useful than exporting.