WELCOME/FAREWELL messages must be stored in listserv filelists.  The
"natural" thing to do is to ask your listserv administrator to create a
filelist for your list and then you can store and revise these files
yourself.  Or the administrator can define these welcome files in a broader
filelist and give you the privelege of updating them.  In either case, your
local admin. has to make the first move.