Basically you need an expansion board. I have one coming up in january, but Joerg makes one as well:
viewtopic.php?f=25&t=3950viewtopic.php?f=40&t=3857If using a V2 board (think I saw one for you in the pile today), you could also do it with a Kisssys expander and a simple 5V BEC since the V2 has the Voltage divider used for battery monitoring and option to supply directly from 5V.
If using a V1 (beta) board, its a bit different and you will need to supply max 4.5V onto Vcom+ and use a external Voltage divider to monitor the main pack.
But take a look at Joergs board. Its very nice and perhaps he can get you one....
I will update the Docu shortly with information about how to use voltage dividers and >1s supplies on V1 and V2 boards.