This thread looked helpful until I got to the bottom and started reading about rebuilding/reconfiguring XFCE. I knew there had to be an easier way to add more workspaces.

This happened a couple years back (same box, Ubuntu 10.04 LTS) and I couldn't remember how. percy's post pointed me in the right direction: "Settings Editor->xfwm4->workspace_count". While I was in there, I saw the keyboard shortcut (tab) which listed alt-insert to add workspaces.

So, it's Settings>Window Mgr>Keyboard tab>Add workspace, which shows <Alt>Insert on my box. I moused over the Workspace Switcher and tried it, and it worked. Where there were once 3 (used to be 4) there are now 6.

Hope this helps someone. Best wishes.

-mz