GroupWise requires eDirectory for management of the system and specific mailbox control. But eDirectory is not required to use or authenticate to GroupWise.
I'm not sure who told you that each eDirectory user object requires you buy a Novell Client license because that is absolutely not true. You only need a Novell user account license if you are going to connect and use the file system or printers served up by the file server.
GroupWise includes eDirectory free of charge and you can create as many accounts as you buy mailboxes for.
You can convert the user objects to GroupWise External Entities if you like but that will limit your ability to manage the client options which control things like views, spell check upon send, etc.
I build GroupWise systems all the time and always use eDirectory user objects and I've never purchased a single NetWare Client license unless as stated above the client plans to use the NetWare file system or printers provided by the server.
I hope that helps.
Scott