I don't know if this help but there is a plugin that allows users to login with steam/battle.net/faceboo/yahoo/twitter and so on.. u can check it here https://www.oneall.com/ It have a mybb plugin and is well documented. Hope this helps you.
Cheers!
MyBB Requests
REQUEST: Steam login for mybb
Submitted by gmodleaker69, 16-07-2018, 11:23 PM, Thread ID: 95724
Thread Closed
RE: REQUEST: Steam login for mybb 24-07-2018, 03:32 PM #2
24-07-2018, 03:32 PM
#2