Yes, both the Denon internal center amp and the center preout will have the same signal and the Denon volume control will control both too. Of course you will have to turn the center speaker to on from the Denon's speaker menu.
A question about Denon's pre-outs?
I have a Denon AVR 3802 and was wondering if I use one of the pre-outs can I also use Denon's amplifiers for that same channel? I was wondering because I had my center speaker hooked up to the Denon's amps and I had a IC cord plugged into the center pre-out to another amp. I did not have a speaker hooked up to the other amp, but if I did would I get sound out of both sets of speakers?