I have come accross the same problem in the past, because due to my ignorance i have bought 2 early doors (1 front and 1 rear) to replace the original ones in my car. Regarding the rear door, it's not too bad. You can drill the early door and fit the pin from the later door. However, you will note that the later doors were reinforced at the point where the pin is fitted, so you have to compensate for this somehow. In my case i have fitted a thick plate inside the door, so that the pin is bolted though this plate.
The front door is more complicated, because the pin fits in a recess that also leaves some room for the stricker that is bolted to the B post. As you will have seen, the early doors don't have this recess, and the early door will need some sort of surgery in order to fit the pin. This was somethinkg that i could not do back then, something that i have left for the future, so i deleted the striker from the B post. Needless to say, this door, the driver's door, is actually like this still today. Obviously it is not as safe as a later door, but since P6s were leaving the factory like this for a few years, i reckon that it can't be too bad.
You will also find that the early doors were fitted with the early round, uncovered locks, while your original doors will probably be the later pear shaped ones with the spring loaded cover. You will need to enlarge the holes in the door skins to the suitable shape, but this is easy enough to do with just a file.