This is a pretty common conversion due most likely to "Keif" doing it to his Teles. It's a good mod and can work well for a Tele that is just a little too "brittle" in all positions. A P-90 is also a great option for the neck of a Tele.
Some Teles have routs to already take a humbuckers in this position, and even a middle pick up but they're mainly Mexican and US ones to save time and money when making different models.
As the Tele has a large scratch plate that covers this whole area it's easy to make several different pick up combination models using one body with several different scratchplates.
This is a Japanese model and only routed for the standard neck single coil pick u so there was a little more work to do but it's not a huge job in the scheme of things. You do however have the take the neck off.
Once I've got the guitar apart I need to mark out where the new pick up has to go. I usually use the front edge (the edge against the neck) as a guide to the front edge of the new pick up.
With the size marked I tape off the whole area with low tack painters tape (even though this isn't a nitro finish) to protect the finish when routing.
If this guitar didn't have a large scratch plate and the pick ups just sat in a plastic surround like a Gibson, the shape of the rout would be different as the mounting screws for the surround fall within the rectangular cavity so you need to keep the "corners" intact.
Once the cavity is routed you just need to cut the scratchplate to fit the humbuckers. This is baically a rectangle with slightly rounded corners. The height adjustment screws comethrough the plate so there are two small holes on either side of the hole.
This is where all your careful marking out pays off as you really want the pole pieces on your pick up to line up with the strings. While most pick ups will still work properly with small alignment issues in this respect, it always looks much better to have them line up properly.
I'm fitting a Brierley humbucker here as I do in most of these jobs where the customer asks for recommendations of good quality aftermarket pick ups. I have personally found Brierleys to be consistantly THE BEST aftermarket pick ups I've heard. Everything of his I've ever fitted has been superb which I can't say about any other boutique or handmade aftermarket pick up manufacturer I've fitted. They're all hand made in Adelaide by one guy who really knows how to make a great pick up. If you're in the market, look him up, you won't be disappointed.