Is there a way to change the template/skin inheritance hierarchy?

Currently you can have:

- custompackage/customtheme 
 - custompackage/default 
     - base/default

Is there a way or a module that would allow more control so you could do (for example):

- custompackage/customtheme1 
  - custompackage/customtheme2 
     - custompackage/default 
          - base/default