Hi Eric,
Soft solder works very well. I've soled many frizzens. You shape the sole to the face of the frizzen, harden it, then simply flux and tin the surface of the frizzen, flux the sole and lay it on top of the tinned surface. Heat the frizzen from underneath until the solder flows and the sole seats it place. Grind the edges flush and you are done. The sole will still be hard. The only trick is to bend the sole a little more than the bend of the face of the frizzen, if it has any bend. That way, when the sole is quenched it tends to straighten a tiny bit. With a little experience, you can usually gauge how much it will straighten. Here is one I did with solder to add mass to the frizzen. The casting was kind of thin and I wanted greater thickness. It has been going strong for several years.
Of course, if a sole is added, you have to make sure it clears the pan fence.
dave