Compatability of NHT 5.1 HT system & Denon 5803

I have a Denon 5803 7.1 Channel The speaker system I am considering is NHT HT which is a 5.1 Channel system
Can I just add the two extra speakers and make this work?

Yes, Try to get the same (surround/rear) model then you'll get the best sound result.
I friend of mine has been running an AV5700 with NHT SuperOnes and SuperZeros in 5.1 mode for a few years.

Depending on the DVD player you're using I would caution you to get a digital coax interconnect with a fuller, warmer balance than a thin, bright one, because the Denon mated to the NHTs can sound on the thin side.

However, these are older models than the ones you're using so it may not be as relevant.