I would recommend purchasing a second center channel from the same package... I assume it's called the Take 1.2? My friend who built up a 6.1 system with Energy e:XL series speakers did this (bought four bookshelfs and two center channel speakers). I think a CC speaker for the surround back channel works well. The horizontal layout of the drivers tends to (IMHO) disperse the sound a bit more, (coming closer to the 7-speaker layout effect) while still being a direct radiator (bi/di polars are *generally* not recommended for the surround back channel.)
Here is the 1.2 for $170, delivered. You could probably find a lower price from a smaller-name e-tailer if you tried: Energy Take 1.2 @ GoodGuys.com
Edit: Woops! They can only ship to Washington, Oregon, and/or California for this product. But at least you know it's available!