Can't assign a Project Manager to an Account that already has an Account Owner

There seems to be a missing piece in the integration of Zoho CRM and Zoho Projects. There is a point where the sales rep (account owner) needs to "hand off" the customer to a Project Manager. The Project Manager will then takeover the account, create projects, etc etc. But the Sales Rep (original account owner) still needs to be able to view the account and his original customer. But apparently for each account there can only be 1 account owner. Right?

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.

Any help you could provide would be greatly appreciated!