I don't know of a way to do 8 channels. I called them once to ask and was also aware of the 6 channels. (Tried to set this up for the Muncie Pre-Invitational this year.)
Actually if you are getting 6 channels to work together, you are VERY LUCKY. Really there are only 5 channels that will work.
From the manual:
"Note: Do not use Channel 31 and 40 at the same time with the 2007 or newer Radio Modems, interference between these Channels can cause intermittent radio communications."
See PDF page #16
http://www.ifirobotics.com/docs/oi-ref-guide-5-8-07.pdf