At design time, GUI information is stored in special files with a .form extension. These files are XML files that conform to a special schema. You can put the .form files into any version control system. A form can be associated with a Java source file.
The GUI Designer suggests two ways of producing GUI forms: you can either create them from scratch, or capture snapshots of the existing dialogs and create GUI Designer Forms from the snapshots. It doesn't matter if the dialogs were hand coded or made with another UI design tool - just capture them from the running application using the Form Snapshot feature.
A snapshot is not related to its original implementation. When you take a snapshot, you thus give up the source code used to produce the form.
IntelliJ IDEA does not provide the means to open the XML source of the .form files in the text editor, and you should not modify these files manually as text.