Clicking the Customize button or right-clicking on a data window and selecting the Customize option will allow you to modify or create new data windows to use with a merge document.
Selecting this option opens InfoMaker on the selected data window. You can then modify the data window and either replace the existing data window or save the modified data window under a new name. When you return to the software from InfoMaker, the new data window can be selected for use in the current merge document.