The only FXG editor I've seen for Flex is by 7jigen, works online or as a Flex app.
Does anyone know of a different one? It can be done in Illustrator I think, but that doesn't really offer a simple export to Flex type option, just gives coordinates.
As far as I know your best bet (if you don't want to use Illustrator) is to grab Inkscape and install the "save as FXG" plug-in. Hats off to JazzyNico for writing what is basically a monster SVG to FXG "XSL Transform" (i.e. XSLT).
How to install:
Drop both files in your Inkscape "extensions" folder.
$HOME/.config/inkscape/extensions
C:\Documents and Settings\User\Application Data\Inkscape\extensions
/Applications/Inkscape.app/Contents/Resources/extensions
Restart Inkscape and you'll find a new FXG entry in the "File > Save As" dialog.
This plugin has been a boon for me and my work, since I've become quite comfortable with Inkscape over the years.
Unfortunately (and this may just be my setup), I've found that whenever I save as FXG, a few minutes later Inkscape crashes out (I really should post a bug report). Just in case you have the same problem, make sure you go to the "Save" preferences and turn on auto-backup. Also, keep in mind that certain constructs in SVG have no equivalent in FXG. Don't ask me what they are, I honestly can't remember.