Can still do NDB overlay approach using GPS in lieu of NDB, so you are technically still doing an NDB approach even without an ADF fitted to the aircraft.
Well I gotta say thats a new one on me.
How do you fly an NDB approach without an ADF? To me, an "overlay" approach is where two different promulgated approaches share the same horizontal profile. If one aid or receiver fails, you might continue on the other if they are depicted on the same approach plate. Different minima may apply. NDB or VOR RWY36 at YBSU is one example.
Just because my jet can draw an inbound line and follow it without raw data available, I'm not aware of any reference that permits it. (Emergencies not withstanding)