Today have I tried Linux Mint 17 Qiana Cinnamon Edition to check out new features. Do not laugh, but most of the time I spent searching for an easy way to add new workspace.
As of today, there is no obvious way to add new workspace using some kind of user application as Preferences/Workspaces does does not contain such option.
You need to use
[L CTRL] + [L ALT] + [UP] or
[L ALT] + [F1] equivalent shortcut to enter Expo mode where you can freely manage existing workspaces and add new ones.