Export:
Get-Recipient -resultsize unlimited | Where {$_.EmailAddresses -like "*@domain.com"} | Export-Csv user_export.csv
Delete all columns except "Identity".
Add a new column named "Alias".
$Recipients = Import-Csv user_export.csv
Foreach ($Item in $Recipients)
{
Set-Mailbox -Identity $Item.Identity -EmailAddresses @{Add=$Item.Alias}
}