There could be an issue, so why risk it. First off, you need to have a basic understanding of welding to avoid either premature failure of the support fitting or damage to the tank wall.
Some suggested requirements;
1. Use an ASME qualified welding procedure specification
2. Use a welder that has some arc time or has welded a coupon
for the weld procedure qualification in (1).
3. Make sure the tank wall is clean of coatings, debris and
grease/oil.
Easiest, safest method is to use a contractor that holds an ASME "R" stamp - repair. And make sure that they are aware that this will be a non-Code repair/alteration, but you want it done in a manner that meets Code.
There could well be problems. How thick is the tank? Will reinforcement be required? Will inevitable distortion cause problems? Is the service environment conducive to stress corrosion cracking? You firs need to address these and other issues before you begin welding.
Any time one welds an austenitic stainless steel precaution is required to ensure that the corrosion resistance of the stainless is not destroyed. To avoid this problem I always list a maximum interpass temperature of 120C.
I would also suggest that you consider using "poison pads" onto which you weld the attachments. Make sure they have rounded corners, and are not welded all the way around so if any cracking occurs at the support to tank welds, propagation of these cracks would be restricted to the pad. You need to size the pad to ensure that you do not place too high point loading on the tank shell, so I would follow Code requirements to size the "poison pad".
In short, the connection detail needs to be designed to avoid shell over stressing - Code or Non-code tank.
