Imran,
SAP does not recommend copying the data model. When you copy the data model the underneath GUID and data elements etc are still referring to the orginal model. I have faced the same problem. I also faced problem in UI. I am not sure if any one else was successfull.
My suggestion will be to create a new data model similar to BP and do your modification/extension as if a custom data model. Please let me know if you succeed in either approach.