I don't see how your idea wouldn't extend your range. You're adding power to the batteries, that's going to extend their range. You're essentially building a "series hybrid" (one that uses ICE to charge the batteries that run the electric motor) rather than a "parallel hybrid" (that uses the ICE to supplement the electric motor as a source for propulsion).

The question is, when running in "extended range" mode, will your car burn less diesel fuel than if you just did a diesel engine conversion on it?

Still sounds like a fun project to play with. Once you've got the conversion done, if you don't like how the generator setup performs, you can always add more batteries and/or modify it in different ways.