The V5 will say the seating capacity is 5 seats, carry more than that in the car and you are breaking the law. If it is possible to add the extra 2 seatsand seat belts you will have to get the car inspected by I guess the DVLAand details on the V5 changed. The insurance company will also need to be informed that the car is now a 7 seater not a 5 seater and since this is a modification they will probably charge you more.
Why not buy a 7 seater in the first place if you need 7 seats.
No, it's not possible to change! Besides all the good info Paul has given, the centre seats are different on the seven seater. They have a sliding mechanism to adjust them forwards/backwards plus of course a tip function so you can get into the rear seats.
As Paul says, if you want 7 seats why not buy one in the first place?