I backupped my .xinitrc, installed kde from the repository on my sd card (ext formatted), made the "ln -s" thing to let use /usr/lib/kde/bin/startkde as xinitrc, and added on top of the new xinitrc the xmodmap line you can find in the backupped xinitrc (the stock one).
Result: I'm using kde with fn key working as expected.
Really amazing