So the alternate way that I thought to do it is to add a custom lookup field to Accounts so that a supervisor could go into an Account, select a system user as the Project Manager at a later time from a Project Manager lookup menu. But there doesn't seem to be a way to add a custom field to an Account using a Lookup that can access Users. It looks like you can access everything from a lookup BUT users.
If I could do that, the account would then have the original Account Owner still assigned to it and now also have a Project Manager assigned to it.
Is there any other way that this could be accomplished? Without this basic functionality, it will be very difficult to use Zoho CRM and Zoho Project together as the Account Owner and Project Manager are almost always 2 different people, who both need access to that account and it's associated contacts.