I wanted to allow users to upload copyrighted content on a private wiki. Unfortunately there is no license for "copyrighted" content, so I tried to create my own entry in the available license list. This seems to be not feasible.
The first problem is that you can not really edit the default configuration of the WikiEditor. You can only create your own configuration hash table which will be merged with the default configuration hash (PHP array_merge function). If this approache works for scalar hash values, this does not work at all for complex ones (and the configuration hash has plenty of them). I think there, that the default configuration should be global and already declared as $wgUploadWizardConfig. It would be easy afterward to customise it.
The second problem seems to be with the string translation. You can't create easily your own license description (in the "licenses" hash) without having to declare somewhere translated strings.
One solution for my own use case would be to patch the default WikiEditor configuration file and add an entry for the copyrighted content... But, I guess there are other people who want to introduce their own custom license.