To add a new RBL (Real-time Block List) for use by CanIt visit Administration->Master RBLs. Enter the details for your RBL there. (The "Online Documentation" link in the upper right corner will describe the options available.)
Next, visit Rules->RBL Rules and create an entry detailing how to use the new RBL. (Again, the "Online Documentation" will describe the options.)
If you create the rule in the 'default' stream it is inherited by all other streams. In other words it will apply to everybody.