I don't believe case types are merged automatically. So if you have a case type for class MYCO-FW and one for MYCO-FW-IMP, only the MYCO-FW-IMP one will be used when you generated new work for MYCO-FW-IMP.
Given then, can you copy the case type from MYCO-FW to MYCO-FW-IMP and then modify the copied one? Is that sufficient?
Once we copy a case type (i.e. Save As) from FW to IMPL, then IMPL is no longer reusing the case type in FW.
We need to do only a minor customization in IMPL (e.g. adding one new stage step or adding a new case-wide local action) but want to continue reusing all other case type behaviors already defined in FW (parent layer).
So far, sounds like Pega7 does not support this type of reuse (i.e. case type behavior).