I HAVE 2 R'off bikes, But know nothing about the frame details on a "Specialized Source Eleven".

http://www.specialized.com/us/en/bikes/multi-use/source/sourceeleven..

a disc brake & belt drive .. the torque transfer would probably be via their OEM 2 piece on the left end.

fitted around the bolt head facing inward on the lowermost ISO caliper, disc mount .

you mean re-lacing the hub in your rim, or buying another wheel Built around the new hub. ?

Disc , you need to buy a new one, 4 bolt ,+ belt drive cog wheel , probably need a new one.
R off has a core , IDK if the Shimano also adapts.. to share the same Belt cog.

I'd visit the better Bike shops in the area.. and see what, they who can see the project,
give as a cost estimate.