Что-то вроде этого должно сделать это:
Set objConnection = CreateObject("ADODB.Connection")
Set objCommand = CreateObject("ADODB.Command")
objConnection.Open "Provider=ADsDSOObject;"
objCommand.ActiveConnection = objConnection
strDomainName = "dc=YOURDOMAIN,dc=com"
strUserCN = "FIRSTNAME LASTNAME"
objCommand.CommandText = "<LDAP://" & strDomainName & ">;(&(objectCategory=person)(objectClass=user)(cn=" & strUserCN & "));samAccountName;subtree"
Set objRecordSet = objCommand.Execute
If Not objRecordset.EOF Then
WScript.Echo objRecordSet.Fields("samAccountName")
End If
objConnection.Close
Set objRecordSet = Nothing
Set objConnection = Nothing
Set objCommand = Nothing