Couple things here.
First - to be clear, the user is linked to an Exchange account no matter what you do - Unity is tied to Exchange mailboxes as the repository for messages so there's no choice in this matter. It can be linked to a disabled AD account so no network access is possible but it's always lined to an Exchange account. Just didn't want anyone reading this thread to be confused on that topic.
You can use GrantUnityAccess to map other login credenticals to that subscriber account such that they can log into PCA for that mailbox - you'll find this in the ToolsDepot on your Unity desktop (it must be run locally) under the Diagnostic Tools section. You can map any number of NT credentials to a single subscriber account if you like.