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.
'11 Ram Quad Cab 4x4 SXT 5.7L HEMI (stock); 48,500m
'06 Pontiac G6 GT 3.5L 220hp; 64,000m
'12 Chrysler 200 Limited 3.6L 283hp; 28,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!