You’d need to add that capability first using other plugins like the one you mention – Members or User Role Editor for example. Then modify the plugin.
But you should be aware of the fact that the code changes will be overriden during each IW update.
After you register and login to the site, you may post all plugin support questions in the Support Forum.
If you need to provide private info, please create a ticket and then reply to it using Set as private reply option.