what you have to do so is create a custom content model. This way you will be able to add properties to folders, and the creation form for the folder will display those properties. The best way to proceed is to use Jeff Potts's tutorial here : http://ecmarchitect.com/archives/2012/01/09/1509.
All you have to know is explained here : how to create the custom model, and how to customize explorer or share to use it.