Hi Niels,
You need to create your's own handler class for this.
Cutom handler class must implement the interface-IF_MDG_BS_BP_ACCESS_HANDLER.
You can refer/inherit from the abstract handler class CL_MDG_BS_FND_HANDLER.
After creating the custom handler class, go to maintenance view(SM30), V_MDG_BS_BP_HDL and create a new entry with data model BP and your class.
Hope this helps you?
Regards,
Mrityunjai