Q: Mac Pro Line In

If I understand your post correctly, you have a headset that has separate connections for microphone and "speakers."

You cannot plug the microphone line into the headset jack and use it as a microphone.

You can get mics that work in the headphone port. However, they have low-level inputs.

How to use Apple Earphones with Remote and Mic with your Mac - Apple Support

I wouldgjet a a USB audio in adopter/mic

Illaass - OP was asking about a microphone-only connection, not a mic combined with headphones.

I just gave an option

Understood.

<http://www.kvconnection.com/product-p/km-iphone-mic-a22.htm>

There is also a mircophone-level version.

<http://www.kvconnection.com/product-p/km-iphone-mic.htm>

If you think about it, most microphones are attached to a headset with a single cable reaching to the aux port. There are many fewer headsets that have a separate microphone cable. The port is designed primarily as an output port, not an input port.