I'm not sure how this works in other browsers, but with chrome you can make desktop shortcuts directly to any webpage Here are instructions on how. So you could make a load of shortcuts and put them in a folder, then import said folder just like you would with any roms, so you would name the "system" something like "websites" and when Launchbox asks for the roms just point it to your shortcuts folder.
Edit:
Actually the easiest way would be to add the shortcuts to a folder then drag the folder onto Launchbox, when the wizard pops up asking what you are importing choose the "none of the above" option.