There are threaded drivers out there to convert a cb to a freewheel. However, the HiStop on the Genesis Onyx does not have the old-shcool aftermarket support for such a thing. While it's possible that one of the early 80s threaded drivers would fit the HiStop (which is essentially a Shimano clone, which in turn is a bit of a Bendix knock-off), it seems unlikely and totally not worth the eBay prices you'd have to pay for such an experiment.
Whatever; even if you're planning to add a disc up front, i think you'll want a rear brake. In this context, i see the obviation of the coaster brake as a downgrade.... or else you'll need to spend more time/effort coming up with a suitable rim brake for the back. And i think the Onyx has powdercoated rims anyway, so.....
I mostly chimed in on this thread to hi-jack it and to ask Frankie if he's ever tried to screw an English-threaded freewheel onto a Velosteel hub.... I have a Velosteel with a threaded driver and put on a freewheel with no problems, but i have yet to run it long-term. Frankie Plekhand, have you tried this? If so, what went wrong?