You can add seat belts anywhere you'd like. Problem is, they won't be considered legitimate by Federal authorities. If you anchor them as securely as possible, they may hold a person in the event of an accident, but they won't be 'official.'
A shop, ruled by insurance and liability issues, wouldn't install seat belts in a new location, but you are free to do it yourself. Utilize the strongest bolts and anchor points you can, but they won't be 'official.'