eCanCrusher is a handy drag/drop tool to unzip and re-zip EPUB files.
The latest version, 1.2.0, has been extended to also support .idml files.
To unzip an InDesign .idml file simply drag/drop the icon for the .idml file onto the eCanCrusher application icon.
A folder or directory by the same name as the .idml file will appear next to the .idml file.
You can then edit the contents of this folder, and once you’re done, drag the icon for the folder onto the eCanCrusher application icon. The operation will be reversed, and a new .idml file that incorporates your changes will appear.
To configure eCanCrusher, simply double-click the application icon. A configuration screen will appear.
Download from here: