I've read the 302 can take an additional input. How would this work with an XLR mic. 99% of the time I use only 3 lav mics but on a rare occasion I need to use a table mic or another lav. Looks like the input is an unbalanced 1/8" stereo jack. What sort of cable do I need to attach mono mic to it. I assume it also wouldn't provide phantom power.

I know there's the 422, but they don't make it anymore (I would have to get a used one), its more expensive, bigger, and heavier

The 'extra inputs' (returns) on the 302 can be routed to the main Left and/or Right main buss, but they are not mic pre-amps, nor do they supply any power, Plug-in or Phantom, however they would work with a wireless receiver that could output an unbalanced line level signal. Riding gain is difficult though due to the small size and location of the return trim pots.
See the manual on how to configure this via the software menu.

I've read the 302 can take an additional input. How would this work with an XLR mic. 99% of the time I use only 3 lav mics but on a rare occasion I need to use a table mic or another lav. Looks like the input is an unbalanced 1/8" stereo jack. What sort of cable do I need to attach mono mic to it. I assume it also wouldn't provide phantom power.

I know there's the 422, but they don't make it anymore (so I'd have to a used one), its more expensive, bigger, and heavier It doesn't work with an XLR mic or any mic for that matter, regardless of connector. Input 4/5 uses the monitor return input which has no preamps and thus is line level only. It also does not offer phantom power. Additional mic level inputs require you add an external preamp such as another 302, a Mix-Pre, or similar, daisy-chained to the 302's mix-in input.

Steve & Rick thx for clearing that up for me.

This might be what your after... Although it won't take mic input level.
Cable Techniques - RX Emergency (http://www.reddingaudio.com/cable-techniques-rx-emergency.php)

Peter Engh Sound at one stage made a P-21 which was a variable 2 into 1 small radio mic mixer designed for the Shure FP 32 / 33 that would also work on a SD 302.
Their current web site doesn't mention it.
http://www.peterengh.com/products.php