In the current 2.4 release of our technology this is not possible.
In de 2.5 release of our technology it is possible to build custom widgets and include these in your model. Custom widgets can be provided and downloaded via the App Store, which will be released along with 2.5.
So, with Mendix 2.5 you can build the widget you need yourself or you can ask someone else to do it for you and to upload it to the App Store.
Mendix is currently working on a tree view widget that will be released with the 2.5 version of the Mendix platform. If you need it before that time you do have the possibility of creating a custom widget yourself by programming one using the Mendix Client API.