@Mohamed Gamil Anwar
Do you mean that you have 6 domains used on other mail servers, now you want to migrate them to Office 365?
About migrating mailboxes from Gmail to Exchange online, you can have a look about this article: Migrate consumer G Suite mailboxes to Microsoft 365 or Office 365 (In this migration way, you need to verify your domain name in Office 365 first)
About the another mail server, if it supported with IMAP, you could migrate mailbox in this way: Migrate other types of IMAP mailboxes to Microsoft 365 or Office 365
After migrated to Office 365, you could modify domain name for those mailboxes as you want, for detailed information, you can have a look about this article: Change your email address to use your custom domain
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