Even if a ticket is on a specific carrier, IF it is FULL FARE, it is endorseable.

There are more variations of expensive tickets these days and few people are buying full fare.

If you are holding a full fare E-ticket for travel or NRT on NW, yes you can walk over to NH or JL and ask them if they have room and if they are willling to except your ticket. It is up to them if they want to except the ticket at face value as covering all your transport or if they want to charge you a higher fare (applying the value of your ticket first).

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YY fares also still exist but I do not know if they can be ticketed wihtout selecting an ailrine. They certainly are endorsable.