Separating emails by commas in Outlook 2016 Mac
Unfortunately, you can't. I had this problem last year with one of my personal clients who wanted to use Outlook 2016 for his email instead of the Apple Mail app. There was no way that I could find to make Outlook switch from semi-colons to commas. I did a fair amount of research then (and did some additional research just now) but came up empty.

In the end, my client went back to Apple Mail.

Sorry I have nothing definitive.