Some people have said that it is possible, but nobody in that thread said they themselves did it.
I have a brand new Mi Box, and just plugged it into the TV. After logging in and getting to the homescreen, when I went to the settings screen at the bottom there was a plus next to my first account's Gmail address. However, whenever I pressed it, it would say something like "Vudu has stopped" and then when I hit okay, it would take me to a blank grey screen.
I then went into apps and uninstalled Vudu and all the other pre-installed apps, thinking they might be interfering with the process, and when going back out, the plus next to my account disappeared. There is no longer an option to add additional accounts at all.
I installed a system update, but that didn't help. My version is now 6.0.1, with build "once-user 6.0.1 MOB31Z 1901".
My next step was factory resetting the box to start from scratch again. Incredibly, the plus is still not present now.
Any ideas or experiences?
Oreo has an option to add another account in the settings, I don't remember about Mm tho.