I don't know if I fully understand what you are asking. Do you want to insert a class between classes, like below?
I'm not sure where your subcases fit into this, since they weren't on the first diagram. Are they classes which inherit from Org-US-App-Work-Proposal? Would those need to change? I assume the "Parent" class isn't a required object, since your existing items aren't children of one. Are you expecting future items to be?
Assuming you're asking if you can do something like I diagramed above, yes. Obviously, anything at the Parent level that overides rules at the Work level will be inherited down at the Proposal layer (and below) so you'll want to keep an eye out for any suprises that might cause. Once you put the new class structure into production, existing items will inherit all of the Parent level rules. You could perhaps use things like circumstancing if you don't want that to happen all the time.