Can an airline lay off its flight attendants and use pilots to fill the role?
When I was flight crew on CRJs for a private operator we got abbreviated FA training for pilots, which was mostly a kind of crowd control course, taught by a contract FA. Their training is about managing a chaotic cabin full or panicking passengers. FA's are more or less under-cover riot control cops who spend their careers working as servers/attendants and are very unlikely to use their actual training.
So theoretically, airlines could offer flight crews FA postions, if they were willing to take the pay cut and were willing to be reminded every day where they are not, performing that undercover waiter/waitress function.  Yes, additional training would be required because airline flight crews don't get significant FA (crowd control) training and a pilot would have to take the FA course.
This is an opinion, but I would say not a difficult call; you would NOT get any takers. Every pilot will take the furlough and go look for another flying job, or change careers.  There is just no way that someone who loves flying enough to jump through all the hoops to get into a flight deck would take that kind of &quot;demotion&quot; (maybe... maybe... one person here or there would agree to it out of sheer desperation, but not enough to be significant).