three kids in auto auto insurance seats, T reg corsa. adapting seat belts?

i have a t reg corsa, is there a way a lap belt can be replaeced (by a mechanic obv) with a full seat belt?

baby three on way, cant get bigger engine cos only passed test last july.
currently driving 1.2 sl corsa automatic (auto licence only) already been quoted cheapest 800 for next year auto insurance.so any bigger engine gonna cost us a small fortune.

hoping i can just change the midde belt for a propper belt so hubby can sit inbetween two kids, and one bub in front?? not comfortable but cheap.