Running a new migration a few days later will import duplicates. At this time it is not possible to bring over just the last 7 days of e-mail.
You may want to consider setting up an IMAP retrieval account under Settings -> My Settings -> Advanced -> Mailbox Migration. you can configure the IMAP account to bring everything into a subfolder instead of your main Inbox. Once the process downloads all of your messages you can drag the items you wish to keep to your Inbox. Once this has been complete, you can remove the IMAP Retrieval account and delete the messages pulled into the specified folder.
I hope this helps.
Technical Support Supervisor