This article will show you how to Export Exchange Server 2010,2013 or 2016 Contacts from a mailbox and then merge them with the contacts of Existing mailbox.
The first step is to Export the contacts:
New-MailboxexportRequest USERID -IncludeFolders "#contacts#"-ExcludeDumpster -FilePath \\SERVER\pst\Name.pst
To Import he contacts and Marge then to the contacts folder use:
New-MailboxImportRequest USERID -FilePath \\SERVER\pst\Name.pst -IncludeFolders "contacts"