I used to have a 2006 family model, and it was designed differently to the standard model - the rear seats split into three so that a single one either side could be folded down to allow access the the rear seat. Also the plastic moulding in the back corners (where the rear speakers are) tapered in more than on the standard one. The sticking point would be the seat mounting -there were recesses with metal rods across for the latches on the third row seat to latch into. Without these you'd have no way of securing the rear seat, unless you're particularly handy at metalwork. So no, it seems if you want extra seats you've got to buy the family version to start with, and that still applies with the latest version -it now comes with two separate rear seats rather than the heavy bench seat the pre-2010 models came with. One of those would be very useful as a spare to keep in the garage for when the grandchildren visit, but no, there is no option of buying just one, you have to pay around £1000 for the family pack which includes both seats and the roof bars, and this option is only available if you order it at time of purchase, its not available afterwards.
