office365 mailbox for one account to a new one.

5 mailboxes export to PST from O365.

Remove the smtp domain from O365.

Add SMTP domain to another tenant.

Create new mailboxes and import PST or import PST to another account in another domain.

U can also use migration wiz to move the mailboxes, but will cost you.

I want to avoid exporting importing.

How much is such a tool?

How do I remove SMTP domain , you mean just to go to the account and remove the domain and add it to the new account?

You're better off doing it manually, all the tools you can use for such a tasks are aimed at the enterprise market and you will not get a good price for just 5 mailboxes. In general, Microsoft doesnt offer any tools to help with the process, just some basic guidance: https://support.office.com/en-us/article/How-to-migrate-mailboxes-from-one-Office-365-tenant-to-another-65af7d77-3e79-44d4-9173-04fd991358b7

More specifically to the domain part, you can use this script to quickly remove any references of the domain: https://gallery.technet.microsoft.com:443/How-to-remove-a-custom-6fe91693