People use different terms for various parts of pressure vessels. Are you talking about nozzle repads?
If you are NOT talking about nozzle repads, then yes you can use AR400 plate for a non-pressure part. See paragraph UG-4(b):
Material for nonpressure parts, such as skirts, supports, baffles, lugs, clips, and extended heat transfer surfaces, need not conform to the specifications for the material to which they are attached or to a material specification permitted in this Division, but if attached to the vessel by welding shall be of weldable quality [see UW-5(b)]....
Doing a quick search, I couldn't locate a P-no. It would have to be an unassigned material, and likely require a custom WPS.
Cheers,
Marty