It's only possible if you know someone who has a welder and knows how to use it. You'd be better off fabricating a third belt into your seat. Several people have done this (on GM trucks). A local business bought a fleet of GM reg cab trucks for service vehicles. They had bucket seats and they had benches installed with a third middle seat with a shoulder belt. A local shop did it for them as dealers wouldn't touch it due to liability reasons. Rear seats will be harder to do a direct swap than fronts. If the current seat has good steel supports it's more than doable if you can find the right person to do it.
'00 Durango R/T 360ci 290hp (modded); 138,500m
'06 Pontiac G6 GT 3.5L 220hp; 44,000m
'12 Chrysler 200 Limited 3.6L 283hp; 13,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!