O artigo Importando contatos de troca de email usando o powershell sugere este código:
Import-Csv .\ExternalContacts.csv|
foreach-object{if (Get-MailContact -anr $_.name) {write-host $_.name 'is a duplicate entry!!!'}
else {New-MailContact -Name $_.Name -DisplayName $_.Name -ExternalEmailAddress $_.ExternalEmailAddress -FirstName $_.FirstName -LastName $_.LastName}
}