Can I connect a microphone to a retina 5k iMac? My 5K retina iMac has one audio port.  On my MacBook pro that only had one port, it was possible, in System Preferences, to state whether to use it as an input or an output.

Is it possible to use the one on the 5k iMac as an input?  It is, perhaps, some weird connector that takes a regular headset or multiple signals, two for stereo audio out and one for audio in?

I recognize that there is a built-in microphone, and there are USB or bluetooth devices that can send in audio data, but it'd sure be handy if I can plug in an audio source as I used to (especially as I have a contact mic that goes to an instrument that I'd like to plug in.)
It should be possible to add a microphone, you just need the correct adapter. See this answer I posted to a similar question - I believe the same will apply in this case.