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
Use Inkscape SVG editor: it will allows you to convert svg to fxg starting version 0.49 thanks to the svg2fxg plug-in.
In the mean time (version 0.48) you can install the svg2fxg plug-in manually as explained by RIAGrande:
Install Inkscape http://inkscape.org/download/
Get the 2 svg2fxg plug-in files from here (from head repository):
http://bazaar.launchpad.net/~inkscape.dev/inkscape/trunk/view/head:/share/extensions/svg2fxg.inx
http://bazaar.launchpad.net/~inkscape.dev/inkscape/trunk/view/head:/share/extensions/svg2fxg.xsl
and copy them in Inkscape extension folder (C:\Program Files (x86)\Inkscape\share\extensions on Windows 7)
You can now start Inkscape and will get a new "File -> Save As -> Flash XML graphics (*.svg)" entry.
I successfully tested it and used the fxg generated assets in on IPhone: it works fine :-)