A list-owner can not add or change another's LISTSERV password.
A list-owner (who remembers their own password) CAN update the
list's password:
1) GET listname (NOL HEAD
2) receive file
3) update headers in the file with list definition keyword
* PW= new_pass_wod
4) send the file back to: LISTSERV with the appropriate
prepended PUT command that specifies the OLD list password
or the list-owner's current personal password
PUT listname LIST PW=old_pass_wd
... rest of headers (no signature block)
5) pray.