How to add labor cost lines to sales order?

We have a country which procures assembled products from manufacturing plant in other country. This country then performs some staging activities - adds a local part, modify paint and performs quality checks. We want to add a labor only line to a sales order to add say few hours of labor cost to this sales order. Price of this line will be zero.

We dont want to enable WIP/Routing for such small activity. Is there a way to add few hours of labor cost to sales order?