There is some bad advice in this thread... You can use your insert to add 2 more channels, however you will be using the tip and not the ring. When tapping the insert (to pull a signal FROM a console), you use the ring. By making a shorted trs cable, you would be pushing a signal at the in AND out at the same time... not good, and I'm pretty sure this spells feedback on most inserts. By hanging a cable 1/2 way out of the jack (I recomend NOT doiing this incase it gets bumped) you will be pushing a signal into the out... not good and useless.
To add a 2nd stereo signal you can simply use a t/s (mono or unbalanced) 1/4" cable pushed all the way in. Keep in mind that you will have to keep your line level source at a max of -3db (same with your mics) and you will have no ability to mix the level of the return feed. An inexpensive line mixer would give you some control over the feed so you could mix it...
Matt