He did it with photoshop. Here's the link to the original posting page.
http://www.virtualmodels.org/bmw-e9-gran-coupe/
Here's the source photo. He made the wheels a bit bigger, stretched the roof a tiny bit and moved the door up and added a second. Of course, engineering such ideas is a whole 'nother ball of wax. Silly to do in practice, since you can just buy an E3 and have a properly engineered and constructed four door vehicle. If you want something that looks like a coupe, buy a coupe.
Yeah, I know, there's a whole industry to help people do things to cars that weren't envisioned by the designers, but it's hard to see how messing with the coupe's lines will result in an improvement. If it's function you want, buy a 4 door car, don't destroy a masterpiece to get marginal improvement in functionality.