Given the proprietary nature of these messaging solutions both on the server side and on the messaging client side, migration away from them is somewhat difficult. Seek professional help. Several organizations have specialized migration software to assist in migrating away from Exchange, Groupwise, and Lotus Notes, including Sun Professional Services and a company called Wingra.
You must export the native format to something more malleable such as CSV or tab delimited files. Then, you can write a script to take this information and create a properly formatted LDIF file for import into the Directory Server.
A basic template to create a user in LDIF format is:
dn: uid=<uid>, ou=people, ...