if a project is created from a project template can the project API's be used to add task?

When our business users create a project they create it from a project template. The problem is that when the new project is created the pm_product_code field is not populated with a external source reference and there no way for our users to enter this at the project creation stage.

Then if we want to use any of these API's UPDATE_AGREEMENT or CREATE_BASELINE_BUDGET or ADD_TASK to update the project details they will not work as all these APIs have the pm_product_code parameter as a mandatory value?

So how do we update the projects created from project templates?