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You need disc brake mounts on the frame, which could be added by a framebuilder/repairer in theory. In practice, however, it's not worth it, as the torsional forces exerted by a rear disc need to be factored in when building a frame, so if the frame wasn't designed for discs you run the risk of breaking the frame.
As @laner says, adding them to the front requires changing the fork (plus a new wheel and front disc brake).
It'd be easier, and cheaper, to sell the Plug and buy a new bike with discs.
disk braks on a charge plug - viable? it's all about the hubs really, innit?
/i don't even braks
/asking for a friend.