Well the easy answer is YES ...and NO! "IF" this vessel is an ASME coded vessel, and after adding your modifications, your are NOT prepared to re-code the vessel (which cost some money) then the answer is NO! UNLESS you down grade that vessel to basically a zero-pressure container, which it remain in that classification the rest of it's life. Now I'm assuming that your adding a connection of some type ...Nozzel or coupling.
However if your adding some type of "support", as long as it's on the skirt area (or support leg) and not close enough to the main body (vessel containment) that the heat effected zone of the weld effectes that portion of the coded vessel, then YES. ...Clear as mud!??