@Spektro:
The way I understand you, what you probably really want is not for the actual ISO image to be editable, but to be able to select more than one directory (tree) to be included when the image is created - right?
This could conceptually be done by having the "Source" button act as "Add source" instead of "Replace source", like it does now. IOW, if you select one source tree first, it is displayed in the content pane. Then you could hit "Add Source" again, and the new selection would be added to the contents. Once you were done, you would "Create image", and from then on, you couldn't change anything in the image.
Some thinking would have to be done about defining the top level directory of the image; it may be necessary to have the user define one directory to create at the root of the image, so the various selections could live within that, otherwise you would risk merging directories with the same name from different sources.
Best regards,
Niels