The short answer is to add the `-Append` option to your `export-csv` statement to stop it overwriting each time round the loop.

Alternatively move the export outside the loop as follows:

    $managers = Import-Csv -Path .\test.csv
    
    $managers|foreach-object{
      $getmanagersam = Get-ADUser -Filter "employeeNumber -eq $($_.'Manager User Sys ID')" | select -ExpandProperty SAMAccountName
      $_|Select *,@{Name='managerUsername';Expression=$getmanagersam}
    } | Export-Csv -Path .\simpletest.csv -NoTypeInformation