So, let me explain how this would work. Quad-channel, like triple channel, should be backward compatible. Depending on how the motherboard manufacturer develops the motherboard, chanells (1,3) will be dual channel, (1,3,5) will be triple channel, and (1,3,5,7) will be quad-channel.
As far as adding an extra stick of Dominator, you could get quad-channel out of it, but it's recommended to have identically batched RAM sticks for maximum performance. You cannot mix memory brand or serial numbers and expect it to work (though it might with a different serial if it comes from a different size kit).