All my Pis are running headless and I don't have a LXDE desktop handy. However according to this:
http://wiki.lxde.org/en/LXPanel you should be able to:
"Basic configuration of LXPanel can be done with the GUI preference configuration dialog. Just right click on the panels, and you can access the config dialog. Please use the GUI instead of directly editing the config files if you don't have very special needs."
.... now having quickly fired up a Lubuntu virtual machine on my laptop and poking around its not quite that simple - sorry
This is the way to do it:
https://unix.stackexchange.com/question ... ar-via-cli
It may seem to be a bit of a fiddle but that is the standard way to define applications for *any* window manager to use ie its a sort of language neutral way to do it. Bigger, more complex window managers give you a funky GUI to do what you want but LXDE isn't called lightweight for nothing.