Migration Email to 365

Hello,

I would like to transfer my email account from our host Siteground to Microsoft's business account. This is because the conventional hoster only has limited email storage and is almost full. The provider Siteground supports IMAP. I have several questions:

1. How do I proceed now? Do I have to transfer the domain first and then can I also transfer the email account in the second step?
2. Once the transfer has been completed, can I create additional email addresses via Siteground or do I now have to create all emails via 365 (which will probably incur costs)?

Thank you!