you can do this thru xbmc as per below
install the Advanced Launcher from the addon repo.
make a script and put in home folder wtih the below (make executable). change "username"with your username
a post with useful information HERE
killall -9 xbmc.bin
sudo -u username /opt/boxee/run-boxee-desktop
go and configure your addon and set the application launcher to the script you just made above. and it should launch.
you can also add this to a favorite on xbmc and then put it on your homescreen (if your skin allows it). I am using the customizable confluence and it works great.
now I have xbmc running as my main program and can launch boxee when I want to see some web content etc.. and the exit and xbmc relaunches.. very nice.