There are three (not too elegant) solutions that come to mind.

1) Use two ICBM's and use the L&R summing input on your sub...

2) ...or you could cascade the Two ICBM's with the LFE input of the downstream ICBM connected to the LFE output of the first ICBM in the chain...

3) ... or, more economically, just set five of your speaker settings to large, and the two rear channels to small, and again use the LFE input on the ICBM to involve the low pass signal from these channels...

I have a ICBM, but I am still a Five channel man, so I may be missing something.