Welcome to Nico! The Terrano's inspired quite a few people here
My guess is that you could install one of those, but it's probably way more work than you're probably willing to invest. The big problem is there's additional hardware behind the rear panel to support the weight of the carrier which is missing if it wasn't installed in the factory. You'd have to locate and install that extra hardware (which may not be possible once the car's assembled).
I don't mean to come across as a naysayer; I just remember seeing a thread where someone was looking to do the same thing with a WD21 and turning green at what needed to be done.
As far as acquiring Terrano parts, I've heard people around here say they've had luck finding them on eBay.
If you're looking to relocate the spare to somewhere other than the bottom (there's a number of reasons to do this), you could always throw it on the roof. There's all sorts of baskets and racks you can install on the roof to complement that route.