I hooked up an external stereo aplifier (Naim NAIT 3) to my NAD T752 surround sound receiver, which works fine.
The NAD manual states that you should use dual-RCA audio cables to "jump" the T 752's L/R MAIN IN jacks to its L/R SURR.B AUDIO PRE-OUT jacks and then connect the surround back speakers to the T752's FRONT L/R speaker outputs, which are then employed for surround-back reproduction.
Does anyone have experience with this? Does it improve the sound quality?
The assumption made in the portion of the NAD manual to which you refer is for the switch to a 7.1 (or possibly to a 6.1) system. Now that you have freed up the (formerly) front channel outputs with a separate amplifier, you could use the (now empty) front channels to supply power to the rear back surround speakers.
Opinions vary as to the usefulness and quality of reproduction with a 7.1 system which will be limited to reproducing (at this point in time) a discrete 5.1 signal. The additional channels are matrixed formats and not true discrete channels as are the other 5.1 signals. The additional speakers can provide some degree of stability in signals which occur in the rear channels, if that is what your system requires. Typically the size of your room and the restrictions on speaker placement and type (monopole, dipole, bipole) and listening position will dictate whether pursuing anything more than a 5.1 layout is worth the effort. In a small room, the general consensus seems to be there is little to be gained with the additional speakers.
Put 7.1 surround format into a search engine on this forum or any of the A/V forums to learn more about why you might want to pursue the addition of more speakers. You can also go to the Home Theater section of this forum for more advice on 7.1 systems.