Can I rig a home theater amp to run in wall speakers?

OK, with the SS6, I'll assume the single speakers are summing speakers as I described above, and he is using 5 of the push button switches, one for each location, 3 rooms, hallway, and outdoors. When playing multiple locations, which one could use a little more volume?

That is hard to say, though I would guess the outdoor speakers will need more amplifier juice, by nature of them being outside.  

Looks like they may not be summing speakers, he sent a photo and 3 pairs are connected as usual, but one line speaker only is connected to one channel. 

Thank you for the help!  

Outdoor speakers would be my guess as well. On the SS6, 3 positions are connected normal, L&R. Then 2 positions, hallway and kitchen, have only one channel connected. Correct?

I do have a couple of ideas, but really need to know the model number of the Marantz to know it's capabilities.

Just heard back from my friend.  It's the Marantz SR5006.

That Marantz has Zone 2 output, as long as you are using 5.1, not 7.1. You can use Zone 2 pre-outs to feed the Rotel to drive the whole house speakers. You can connect all the speakers to the SS-6 and the Rotel will probably have enough power to drive them, although I would check to see if it can handle the 5 ohm load from the SS-6. In-wall and outdoor speakers are usually pretty efficient so the Rotel should be OK if it has 100W output. You will probably need analog inputs to use Zone 2.  Also, check to see if the speakers are all 8 ohm. That many 4 ohm speakers would definitely be a problem

You can certainly use a simple RCA splitter to separate the signal in order to use 4 channels of the Rotel. There can be some minor impedance issues between the Marantz and the Rotel doing that, but the effect is pretty low and should not be an issue for whole house speakers.

The biggest problem will be volume control. You are almost certainty going to want to be able to change volume for different rooms. You could use impedance balanced speaker selector boxes like the Niles SSVC-6. Make sure such a box has impedance matching. For example, the Monoprice ones do not, I believe.

So, I would check the speaker impedances, the impedance the Rotel will handle and think about volume control.