Are you familiar with welding CA-6NM and 9Cr-1Mo materials? I would presume that the nozzle be fillet welded to the CA-6NM pump case. You did not mention the thickness of the pump case or the size of the nozzle.
What I would do is to butter the entire weld prep end of the 9Cr-1Mo nozzle and perform a separate PWHT before you weld it to the CA-6NM pump casing. Use preheat for buttering to the 9Cr-1Mo base material. The butter layer should be 1/4" thick to assure that welding the nozzle to the pump case will not affect the Cr-Mo base material. The filler metal should be 410NiMo for buttering. This is a low carbon martensitic stainless steel filler metal that would be compatible with Cr-Mo and CA-6NM.
After you buttered the nozzle, weld it to the CA-6NM pump casing using the same filler metal with no preheat. Once this operation is completed, perform a local post weld heat treatment of the nozzle attachment weld at 1150 deg F for 1 hr/inch of weld deposit thickness (not base metal thickness of the pump case).
I would still insist on qualifying a weld procedure before you perform this work. I would also perform a wet fluorescent MT after the nozzle is welded to the pump case.