"As I was reading one of your articles on phantom power, I read over something that said you should not have line level outputs plugged into the mic input of a mixer (for the reasons mentioned about the danger of phantom power to a line level device as well as the mic level input not being setup for a line level source). I'm using an older Yamaha mixer for live sound, and we have three wireless mics with three receivers and the mixer doesn't have line level inputs. I was wondering is it possible to use the insert input for the channel? Would it work without an input into the main XLR input of that channel?"

If the channels have insert jacks that would be an excellent way to inject a line level signal. Inserts come in many forms. Some have one TRS connector where the send and return share a common ground or shield. The ring or tip can be the send or the return. There is no standard for this, though usually the tip is the send. Others have individual inputs and outputs for send and return. Inserts also don't always occur at the same point in the channel signal path. The most common place is right after the mic preamp and before the equalizer, but I've seen mixers where the insert is the last thing before the fader. That would be unfortunate in your circumstance because you wouldn't be able to apply EQ or use effects sends on those sources.

If, for some reason one must use a mic input for a line level signal then all you can do is pad the signal down enough that it doesn't overload the mic preamp. You can buy or build pads for this purpose. Then you just have to pay strict attention to Phantom power and make sure it stays off on that channel or verify that it will not hurt the line level gear feeding into the mixer.