Tupical, aggregating/centralising other mail services is NOT the use case for MIAB, which is about running an independent, secure, mail server for a domain you own.
Of course that does not mean that it cannot be done, but you would have to build that yourself:
1. Add text boxes to the web interface
2. Store username and password somewhere securely
3. Have a daemon (for instance Fetchmail) connecting to other mailservers like Gmail
4. Receive incoming email over pull/push
5. Deliver this to the user mailboxes.
But even then, if you also want users on MIAB to send email out for that Gmail account, you will have to deal with Gmail SMTP servers settings. Sounds all trivial, but takes work to do all this well. But feel free to branch and develop this! Who knows it may end up upstream if Josh agrees it is a must-have.