I can think of the following,
1. When you click on the submit button of the child window, invoke a action class. In the action class create a method to map the values from the child windows form bean to parent windows form bean. Once the mapping is done, forward your request to the parent window which would have the values of your child window as well.
2. Have hidden fields in your parent window for all the fields that you might have in your child window. Use
Java script to populate the values down to the parent window from the child window as soon as the onclick of the child windows submit button is triggered.