That's quite interesting to me what you are expected to do, because I had to manage a documentum migration onto a solaris cluster in the past (I guess in 2001).
Your login within the documentum portlet provides a session on the documentum side, what you need to have is a session on the portal side too.
In order to setup a portal session it needs access to the users and roles - ideally they are the same as used by documentum (ldap?). If that's not possible may be the synchronizing modules as mentioned in chapter 18 may help you?
On the Documentum side, the portlet is configured as "trusted". A Documentum superuser makes the initial connection, and compares the Jboss-supplied user principal with the users in Documentum. Assuming the principal authenticates with Jboss, and matches a Documentum user, the Documentum module creates a session for that user.
The trouble is that I don't know how to set up the authentication to pass the logged-in principal to Documentum.
I'll peruse that link though. Thanks.