There are shops that fit them and sell the belt extension kits.
Retrofitting iso isn't legal. Unless engineered.
Do you mean back seat or the jump seat in the cargo area?
The actual cargo are seats may be in the car but unless the ADR plate states it is a 7 seater they are illegal too.
Plus, a 240 will use the cargo area as a crush zone , old cars just are not safe compared to ANYTHING newer.