$objs =@();
$output = Import-csv -Path C:UsersDanDesktopExternal_ID_Test.csv | ForEach {
$Object = New-Object PSObject -Property @{
        Employee_ID = $_.Employye_ID
        Base_Cost_Center = $_.Base_Cost_Center
        Entity = $_.Entity
        ExternalID  = [String]::Concat($_.Entity,$_.Base_Cost_Center)
               }
    $objs += $Object;
}
$objs
$objs | Export-CSv C:UsersDanDesktopExport_External_ID_Test.csv -NoTypeInformation