Route mail to a mail enabled contact

I want to create mail enabled contacts for 500 mailboxes in a company were aquring and do away with their mailboxes.  Then create mailboxes here at my domain but any mail sent to their old company email have that domain route the mail using the mail enabled contact so I would create the contacts add the external address as "newcompanydomainEmail".  Then  add a smtp address to the contact for the "existing domain email".  Correct?

Im using this script to create the contacts with an CSV that gathered names and mail address .

 Import-Csv “contacts.csv” | ForEach {New-MailContact -Name $_.Name -Firstname $_.FirstName -LastName $_.LastName -ExternalEmailAddress $_.ExternalEmailAddress -OrganizationalUnit “Domainname/OUname”}

How can I add the smtp address using a script and where do I make a rule or whatever to have the mail rout through this contact so I can delete their mailboxes on the olddomain?