Looking at the specification for this receiver, it is a serious piece of kit – the last thing you want is to blow an output channel due to overload.
The amp has 7 outputs; from what you say you are using all 7 (5.1 surround and 2 for stereo).
If you look at the spec, the unit has 2 sub-woofer outputs, however these require powered sub-woofers. The sub-woofer signal is not stereo, but the same signal to both outputs. The receiver does allow the low frequency cut off point to be set up to 250Hz, but still very bassey.
My advice is to feed the zone 2 L & R to a suitable power amp and use that to feed the extra room speakers (see page 15 of the owners manual for the output location).
You could simply add an additional pair of speakers in parallel with any pair at the receiver output, with the proviso that the resultant speaker impedance is at least 6 ohm – otherwise you risk blowing the output.
https://usa.denon.com/us/product/hometheater/receivers/avrx3000