Radiobuttons per definition only support one selected item per set.
What you want to use is checkboxes, only the CheckboxSetSelector doesn't support enumerations, and neither does the framework.
What I'd advise you to do is create the enum values as objects and use a referenceset to the object the enum was on originally. Now you can use the CheckboxSetSelector and it looks just as if it was just a normal enum. This would also let you be more flexible as you can just create more enum objects from your application. It also lets you use constraints more easily.
No that's not possible and would be undesirable, an enumeration can only have one value at a time. Maybe you can solve your problem with a reference set?