Alloy 20 (E/ER320) is an austenitic SS (sorta) designed originally for suphuric acid service. Its chemistry is actually quite close to Alloy 800 though it is Cb stablized and contains Cu, hence the sulphuric resistance. I believe its base metal specs are actually found in ASME Section IIB with the Non ferrous materials (because of high nickel) although the filler metal is the AWS SS specs.
You can weld it with a variety of fillers in dissimilars depending upon your service. E/ER320LR is the common choice. Nickels have also been used though I don't believe this is desirable for most applications. And perhaps even a 316L or a 309LMo. It really depends upon your service.
Chuck will certainly have a lot more info on this one.