Well, the file already is a set of emails (a "mbox" format file). To copy its contents to IMAP, you can run mutt -f /var/mail/root
and use the Shift+C (copy) or s (move) commands, and enter imap://user@mail.example.com
as the destination.
To convert the mbox file to Maildir format (assuming your IMAP server uses Maildir), you would do exactly the same but specify a local directory path as the destination, e.g. /tmp/rootmail/
. (Note that the final /
is important – without it, you'd just get another mbox file.)