Will this be a butt weld? If so, no WPS will allow to weld directly a .025 inches thick plate to another plate .250 inches thick, no matter the material.
In the point where the weld will take place, the two edges must have the same thickness. So, the carbon steel edge will have to be ground to match the stainless steel edge thickness by means of a grinding wheel. The taper of the ground section may be 3:1.
Giovanni S. Crisi
Sao Paulo - Brazil
Typically I've seen stainless electrodes used to weld stainless sheet to carbon steel. That puts them in the stainless code, since it's the only one that handles stainless electrodes. The stainless code has an appendix about welding SS to CS.
There are lots of other past posts on this forum about welding stainless to carbon steel.
Hg