You would technically be able to add more inputs with another device in conjunction with the AudioBox USB by creating an aggregate device in the Mac OS X or using ASIO4ALL as described in the link you've provided. Since this is not a supported use of the AudioBox you would need to go to the forums for assistance with creating an aggregate device to do this.
However, for ease of use and setup I would personally recommend upgrading to an interface that has expandable I/O such as the Studio 192 or Studio 192 Mobile which have ADAT available.
