How to mix High-Z and Low-Z audio signals? (Sum should be High-Z)
You could do something like this if you're not afraid of op amps. The microphone gets its high impedance, and if it needs higher impedance, increase R1. I have the iPod a 330 Ohm load because I seem to remember reading somewhere that's a good starting point. The output impedance is 10k, and can be increased if necessary. This circuit has a gain of 2, so if you need less, decrease R6, and if you need less than 1, switch it to an inverting configuration. Trying to sum these inputs passively is going to require a matching network.