Hi Fabrizio,
How about creating a separate Invoice for the Service Material and using Invoice List-Would that be okay to your requirement.
Alternatively, how about updating the Service Material in the Delivery and using it in the Invoice. The copy control will have to be handled to take care of the preeceding document reference (they can have the same Sales Order number and Item number as the one immediately above them). We have used this alternate solution for a similar project requirement.
Regards,
Shashi