I think you are jumping the gun. You don't want to know how to "change four lug to five lug" you want to know "how to change an Argentinean Taunus GT from four lug to five lug". That is where you need to start, getting in touch with people who know this car. It is not built or even distributed here, few people if any on this forum will know the car even exists (I didn't) let alone what parts would be needed to make the swap.
It is possible this car is based on another car we may know here in the states, but I couldn't tell you which one. You may be able to get bolt on parts to swap it over to five lug. But then again, there may be no such parts and you would have to remove the the axles and bring them to a qualified machine shop and have them redrilled and install new studs in the holes.
This may not be possible, depending on the size and shape of the axle flange.
Cesar, I suggest you slow down a little on your plans and get to know the car a lot better before you jump into it. There are LOTS of super well handling cars with four lug wheels, there are LOTS of custom wheels for four lug.
Just get in touch with people who hot rod these cars and you will see all kinds of stuff that can be done.
Don't be disappointed if the your grandpa tells you he can't get it to you. I had a custmer get a couple of cars out of Argentina and it was very costly. The government wants a big chunk of money in fees, many times its more than the car is worth.
I did a search on the Taunus and it appears it is German! Taunus forum (click here)
or check this one Ford Capri/Taunus forum (click here)
It appears the car has the same "Platform" as the Mercury Capri that was made in Germany but sold here in the states in the 1970s.
If you can't find an engish forum, use a translater like Altavista.com has and post a question. What the heck, you have nothing to loose.
I know one thing, a V6 Capri was a little fireball. I remember taking them off my dads used car lot and smoking the tires with them.