How to add a subwoofer to my system?

you connect the sub to your amp "pre-outs" not to an additional speaker post

You can even add a smaller subs to right and left pre outs as Larryken stated. I added 2 small boston subs my cdm9nt's for some extra bass.

I went with 2 Y jacks going out of R+L preouts.
one pair cables from each y jack(sub+amp,right side)left side same thing,speakers on large.

Find a sub with speaker-level inputs and tap the sub off of your speaker binding posts (either from your amp or from one of your speakers).

Look for a passive sub, non powered and use the B outs as a start. I have some RA Labs (Roy Allison) passive subs, that have a pass thru of the highs, but I put an 8ohm resistor on the out of the sub. Great sound. Fantastic performance, equals NHT sub which I use on another system.

I use RELs driven from the speaker outputs. If you adjust them properly you will not need to turn them off ; you will not know they are there if the music doesn't need them. If you can "hear" the sub it is set too high, subs should be felt and not heard. Set the crossover at it's lowest position and work up, when you can hear the sub back down a little.

Look for one that allows flexibility. Phase controls, a good xover range, volume, good power amp, high and low pass filters, (for now and later), and the right size for your room.

I have a 14x21x8 roughly, sized room... I've had 9, 10, 12, and now a 15 inch sub in it. All the speakers running with it didn't go below about 35hz or so, some even fell off sooner at 40-45, or more... if I could afford it I'd have one more likewise 15, or a pair of other 15 inchers, but a pair do seem to be the ticket... although one does a pretty dog gone good job.

I've always ran subs with RCA ICs out from the preamp and/or processor. I like sealed units which are front firing more so too than downfiring and ported subs.

Take a gander at Rel, HSU, Velodyne and SVS, in no particular order.

When it comes to subs... size does matter and depends greatly on the room's cubic footage as well.

Good luck