I doubt it. When these 10 rounders were made, it was illegal to modify them to hold 12. So, Sig helped prevent us from doing something illegal by using this design. Furthermore, aftermarket manufacturers would not have begun manufacturing a part for the purposes of making illegal conversions. Finally, once the ban was lifted and you could get a Sig Factory 12 rounder for the same price, there was no longer any incentive for a company to manufacture the conversions.
What you describe (floorplate that adds capacity) does exist...MecGar makes them...but they are designed to replace normal capacity mag floorplates and not these 10 round blocks.
My advice: buy some 12 round MecGar aftermarkets or factory mags for self defense; keep your 10 rounders for the practice range.