First, this is way off topic for this forum. You should post WindowBuilder questions/comments to the
WindowBuilder forum.
Second, the behavior you describe in WindowBuilder is quite intentional (i.e., not allowing you to create loose components that aren't part of the component hierarchy).
Third,
WindowBuilder is an Eclipse.org open source project, so you can enter a feature request in the
Eclipse Bugzilla (Tools > WindowBuilder) and, even better, submit a patch to allow the behavior you want.