There are several ways to do this, depending on your exact needs
You can view a user's 'Inbox' messages by clicking on the message count on the Users list, then select the messages, choose the target user, and press the Move or Copy button
In version 5 you can view messages from folders other than the 'Inbox' folder, but only move them to the target user's Inbox. In earlier versions of VPOP3 you can only view & move messages from the 'Inbox' folder.
This only works with VPOP3 Enterprise.
The easiest way is to configure the email client to access both accounts using IMAP4, and drag the folders/messages from one account to the other. Note that this will require the email client to download the message from the first account, then upload it to the second, so may be time consuming for moving large numbers of messages.
An alternative is to share the target user's folders with the source user, and then drag the messages across within the one account. This is a lot quicker since the email client just issues an IMAP4 'COPY' command to VPOP3, rather than having to download all the messages, then upload them again.
Note that this is only for very advanced users, and we do not offer support for this method - if you mess up your message store, or lose messages because of doing this, that is your own responsibility. If you want us to do this for you, then we are able to do it remotely as part of a chargeable support call (£20 + VAT)
This method just allows you to move entire folders between users.
Note that the instructions below are for VPOP3 versions 3 to 5, and do not apply to earlier versions, and may change in later versions.
Do NOT skip the Backup VPOP3 step - it is very easy to go wrong doing this, and you wouldn't want to lose all your messages!
You may need to delete and re-create the IMAP account in the email client to have it load the new folders correctly (or have it reload all the folders if the email client supports that option - Outlook doesn't)