without the user who owns the original mailbox being aware?

I don't know if there is a way to do this, but I'd be very grateful if anyone can give me some quick help here since I've been asked to get this done ASAP.

What we want to do is copy thousands of messages but not an entire mailbox to a resource mailbox we created. I know we can do that by creating a shared folder, but my struggle here is that the original user can't be aware that the messages are being copied. We also cannot obtain that user's password in order to set them up as an IMAP user, which is another method I understood could be used.

What I tried to do but did not work was this:

1) Proxied into User A
2) Clicked Window and chose "New Main Window"
3) From the second window I proxied into Resource B
4) Attempted to copy/drag messages from User A to Resource B

I do have write access to all folders on Resource B.

Is there any way to accomplish what I need? We do have a Retain archive of the user's mailbox, but my manager really wants the messages in that Groupwise resource's mailbox. There are thousands of them though, so forwarding them manually just isn't an option.

Help? :)