I plan to do this conversion on my Nomad MK2. I don't know if anyone has done this before..
Why not just order a Nomad Mk3 that already has a splitter, and a better elliptical bottom-bracket design? (Or, if you are on the continent, one of the other touring/bikepacking frames that comes with a splitter.) You are already looking at hundreds of euro to install all the Gates belt-drive components, and people who can afford that may not find the price of a new frame too much to bear.
As well, i need to choose the adequat sprocket in order to get the same demultiplication that i've actually, either 36X16 and the correct belt length. About to respect a correct chain line, i'm lost. Any information will be welcome
This is why it is best to have the Gates drive installed by a shop that already specializes in belt drives (and they often specialize in splitters, too). They are already used to calculating all these things for customers.
While the other poster recommends Veer, if you plan on actually using the belt-drive Nomad as a serious expedition bike, then IMO it is advisable to go with the Gates belt drive, which has over a decade now of proven reliability all over the world.