You might want to look at the Flashexpander plugin which you'll find in the Extensions section
-- Green Button - Green Button - Extensions -- look for and install the Flashexpander plugin, then use the plugin to setup the new file system using an SD card or other USB disc attached to the box.
Once set-up, in order to check how your file system looks after the changes
-- Menu - Information - Devices --
Your /usr space should show a value commensurate with the size of the Card or USB stick being used