Well long answer ...
1) Your motherboard/cpu only support dualchannel,hasn't got to do with how many sticks you use.
2) adding ram may give problems,it's not guaranteed to work. Read from the following,
http://www.tomshardware.com/reviews/ddr-dram-myths,4155.html#p4
"Just Adding More Dram".
In reality it's about trying,make sure to get another set as close as possible in specs,really if you can the same sticks you use,but still no guarantee it will work.We could make maybe some adjustments to voltage and maybe other settings to make things work together,but that would be after testing initially.
If you can would getting a 16gb set (2x8gb) be the best and sell what you own to recuperate some money.