I’m a subscriber to Office 365 and would use their mail service (whatever it’s called these days) but... I can’t bring my email address - which is on a grandfat

I’m a subscriber to Office 365 and would use their mail service (whatever it’s called these days) but... I can’t bring my email address - which is on a grandfathered free ‘Google apps for your domain’ account. It’s a bit silly, considering I’m paying for this and can’t use it, but am getting Google’s email hosting and spam filter for free.