A issue of confusion here may be there are now two types of FFCs -- the original type and the new, residual, ETC-like FFC. The former can not be used for another passenger, the latter can be.

Back to the original request, you can not add a passenger to an existing PNR -- existing or cancelled, especially if the fares are different. The only solution is to create a new ticket, both at the existing fare. Even if you could get an ETC, your new tickets would be at the higher fare.

But why do you want a common PNR, can this problem be solved in a different way?