adding a subwoofer to my 2 channel lanai system

It appears the MA6500 is stable to 2 ohms, but still best make sure there won't be a problem.

What is the impedance of living room speakers, and does the .1 subwoofer have it's own power?

Here is an example to show you a proper way to install a single in-ceiling subwoofer, assuming you want it powered by the MA6500.

OEM Systems

Use the dual voice coil model of the woofer and 2 crossovers, one for each channel, then wire using the Crossover Option 2 diagram on page 16 of the manual.

All of the above can also be accomplished by using a SpeakerCraft 8.2 BAS. This in-ceiling woofer is a dual voice coil and has the crossovers mounted on the back. It also has a phase switch and the crossover is adjustable.

Just thought of another issue, it will be important to choose a woofer with the same sensitivity of the other in-ceiling speakers, although a 1db difference will likely not matter.

Since living room speakers are 6 ohm with a powered sub, impedance will not be a problem.

As far as the SpeakerCraft and OEM Systems, these were just shown as examples of how to properly do what you were asking.

Another option would be a ceiling mounted woofer with an external subwoofer amp like these,

ICS11 Ceiling Subwoofer

M3 Subwoofer Amplifier

The subwoofer amp will need to connect to the MA6500's L&R pre out.