Migration from pop to imap

We recently installed a suse std. server as a mail server to replace an older machine which delivered through pop. We are trying to get the users to switch from pop to imap. The problem we have is that some users will want to transfer their old messages to their imap folders. We use mozilla as mail client and when we try to copy the old emails, the transfer fail with either one of these messages: "The current command did not succeed. The mail server responded: Message contains bare newlines" "The current command did not succeed. The mail server responded: Message contains invalid header" Is there a way to import old pop mailboxes into these imap accounts that avoids these problems? -- Rafael