使用NHibernate在中间表上使用附加数据

I'm working on a project that is used to input information about a product into a document. One section has a form that can be filled out as many times as needed for a particular product. The information on this form is then available to be shared on multiple related documents. A typical many-to-many situation. The difference is, on the intermediate table, I want to store specific information for that form entry for that specific document.I'm working on a project that is used to input