You'll need to find a shop that will accept a non-roster single action from an out of area shipper. Nothing illegal. When done correctly, they'll be receiving a functional single action revolver and single actions don't need to be on the roster to be sold in California.
This isn't a "loophole". They'll be receiving a true single action revolver.
Once it's been DROS'd and you have taken your ownership after the 10 days, there is nothing illegal about converting your single action revolver into a double action. The fact that the process only requires a double action sear, a spring, and a pin is irrelevant.
You're seller should ship you these parts directly when they ship the revolver to your FFL.
If you're not afraid to crack the side plate on a revolver, it really is dead nuts simple.
There are a few caveats regarding the size of the pistol. 3" barrel minimum and I don't think J frames with 3" barrels are over the 7.5" OAL minimum.
There have been posts about it here on this forum and on Calguns.