Link another account to an existing Office 365 mailbox

Dear.
We have two forests. Today, our users are in Forest A and we have a federation with Office 365. We want to move the accounts from forest A to Forest B. So, we need to two things:

1. Create an additional federation between forest B and Office 365.
2. Do something in Office 365 to link the account from Forest B to the existing mailbox.

For point 2, are there command line, PowerShell commands we can use to automate this? If so, how.
Does someone has a link to a good article or document describing this procedure?

Thanks in advance.
Regards.