Install the Ipswitch Mail onto the server you need. Create a link between the GroupWise system and this external system via one of the groupWise gateways. You may need to look at the SMTP or the API gateway. The SMTP shold help in maintaining a basic format of the email, I'm not sure about attachments. You may also need the API gateway. The API gateway allows you to control groupwise at a very low level. With it you can extract mail, attachments, reformat them and send them onward.
Actually, why not leave the GroupWise and Novell server running. You can still you Outlook as a POP or IMAP client. But get this, there is a dll that will allow OutLook to connect directly to the GroupWise system. So you can have your Outlook and GroupWise also.
The big problem with running dual email systems is the synch between the systems of user ids and passwords. Set the GroupWise system to be IP only mode. This will prevent users from trying to map a drive to the mail box. Ensure thesettings on the POA are to allow IP only connections.
If you need more info, I have done large scale migrations from GroupWise and to GroupWise. Please feel free to post another question or send me an email.