Create a custom Dynamics Navision 2009 webservice

We need a new webservice (based on a new codeunit) that changes a single boolean value in the Purchase table. One of our clients has a custom boolean field in the Purchase table that makes an invoice read-only, the default page-based web service doesn't allow that field to be checked/unchecked when creating a new invoice or updating an existing invoice via the webservice, but it can be done from the user interface. We are integrating with NAV and need to be able to also check/uncheck this column through the web service. A codeunit/webservice is needed for this purpose. Developer must have proper licensing to add codeunits. Navision version is 2009 SP1.