I thought Magic Lantern had already done this with the 5D3??
Dual pixel tech is a hardware feature. It cannot be added with firmware. Canon would have to have actually manufactured the 5D III sensor with dual pixel technology at the time they released it in order to add the capability with a firmware update later on.
Given the work that ML has done, if that was the case, I would have expected them to have figured that out by now, what with all of the things they have been doing with the 5D III lately. Given that they have not, it seems unlikely that the 5D III sensor was actually manufactured with DPAF tech.
If Canon does offer an upgrade, it would be a "Send your camera in and we'll replace the main board with one that has a DPAF sensor". And, that would probably cost a pretty penny, too! They may release an interim update to the 5D III, like the 5D IIIdp, that includes just a new sensor and no other model changes. Canon has done small interim camera model updates in the past, like the 1D IIn.