bridging integrated amplifier

If you want both channels output through a single speaker rated at 8 ohms, you could just connect it conventionally via one set of speaer terminals, then set NAD's speaker impedance selector to 8 ohms, and then press the "Mono" selector button on the front right panel of the receiver, to merge stereo signals into a single mono output.
Do you actually need to double the power output, anyway?

Rome,
One additional thought - if your single speaker is able to be biwired - that is, has separate sets of connectors for low-frequency and high-frequency ranges, you should actually be able to connect both "channels" of your receiver while in mono mode. Remove the speaker's jumpers and then conventionally connect "left channel" outputs to one set of terminals and then "right channel" outputs to the other. Both channels would actually be outputting in mono mode. Not sure whether this will actually deliver more power or better dynamics but it may be worth a try.
Best,