I've got a few sennheiser lapel mics that terminate on 3.5mm locking connectors and I wanted to be able to use them with a fixed cable XLR.
Just a simple straight-through adapter is not possible as the most lapel mics are electret and therefore need a small bias voltage (not as high as 48V phantom power). The following diagram below is how I derive this bias voltage and pass through the audio. Bare in mind this output from one of these mics is technically unbalanced and a transformer to correct this would not pass the phantom power - you could in theory simply use a small battery instead.
(For best results, try to not use an electrolytic capacitor for the audio-bypass.