How can i Convert multiple User Mailboxes into linked mailboxes

In my Senario I have Exchange forest where multiple domain hosted and connected through MPLS. Now company want that every active directory forest users should have access on their mailboxes, but all mailboxes should be linked.

I have around 6000 Users mail boxes

I want to convert around 4000 users mailboxes into linked mailboxes

Is there some way to convert bulk of mailboxes into linked mailbox.

I need assistance to full fill our client requirement.