You can be assured that someone, somewhere has done it. I don't doubt that. I've never heard of a conversion kit for sale. The main reason for this is there's not any market for one.
A company needs to be able to sell the stuff it makes. If no one wants to buy, no one will make it. The low price of the Saiga, which is a very similar configuration to the SKS, and already takes detachable mags, kinda puts the kabash on a conversion.
You'd need to convert an SKS with enough US parts to meet the regs, same as if you were converting a single-stack ten round AK. Then you'd actually need to make whatever parts you need fo the mag. Then do whatever gunsmithing you need to get all that to work together. IIRC the receiver on the SKS/D (detachable AK mag SKS) had to be modified from the original physically. Add the cost up for all that, and it's much cheaper to buy a Saiga.
Not knocking your idea. A Saiga is definately not an SKS, so if you want to make an SKS take AK mags, have at it. But in the end, it's probably cheaper to buy one of the Chinese ones you've already had in the past and change the stock to something more human formed. If you just want a "conventional" 7.62x39 (i.e. no pistol grip) then the Saiga would be the way to go.
Ross