Hey everyone, I think I found a fix for needing to know all user's passwords. Being that I can't write scripts like other people had said they've done to make imapsync easier, I did some more research and found out about --authuser1.
Check out this thread. It explains how to migrate users without needing to know their password at all, authenticating against 1 user account (most likely domain admin).
You'll need to scroll down a bit to find the string that worked for this guy. And it looks like authmech needs to be login, not PLAIN. I hope this helps!
*Edit* This doesn't work the way it was explained. Still need all user passwords. Mod can delete this post if you like.