I thought I'd add this. I found that it is possible to use an EXTERNAL MICROPHONE with an iMac Pro (purchased late 2018). I was looking for a solution for a long time. Nothing obvious was listed on the Apple web site. According to the ports specification there is only a 3.5mm jack for HEADPHONES only.
All that is required is a MICROPHONE/HEADPHONE 3.5mm splitter. I thought I needed to buy a USB audio adapter for around a hundred bucks. Fortunately this is not required. I hope this splitter saves you a bit of time and money. I use this splitter on the iMac Pro with various RODE mics (with and without pre-amps) and they all work fine.