Moving emails isn't easy, because Plesk uses Maildir format while DA uses mbox format.
If I recall correctly, one of the articles in Linux Magazine (a UK publication), November 2004, mentioned a program that could be used to move emails from one IMAP server to another. I don't know if emails so moved would be available to POP on DA or not, as I've not tested it.
This may not be a problem for you; if you've got users with mail on the Plesk server that needs to be moved, they're probably using IMAP (or on Plesk, Horde Webmail, which is an IMAP client).
DA doesn't include Horde by default; your IMAP webmail clients will have to use Squirrelmail unless you install Horde yourself or have someone do it.
Jeff