Import-CSV .\BulkAdAccounts.csv | 
    ForEach-Object {
        $ADUser  =  Get-ADUser -Filter "sAMAccountName -eq '$($_.UserName)'"
        $manager = (Get-ADUser -Filter "sAMAccountName -eq '$($_.Manager)'").distinguishedname

        if ($ADUser -and $manager) {
            $ADUser | Set-ADUser -manager $manager
        }
    }