Add a new field in an existing Form and corresponding business object

Hi,

I have seen examples of adding new forms and creating User Defined Objects. Is there a detailed example on adding a field in an existing Form and its corresponding business object, and persisting the same to the database, including user input validations?

Example, I want to programmatically add a new field in the detail section (grid / matrix) of a Marketing Document, and save it to database and read it back when the document is opened.

Thanks.