The ADD1 template, by default, is a two line message:

.re owners
&WEEKDAY, &DATE &TIME

You have been added to the &LISTNAME mailing list (&TITLE) by
&MBX(&INVOKER).

.im $SIGNUP

which is appended to the $SIGNUP template.  Therefore it is likely that you
are looking for a separate mailing for ADD1, when in fact it is
incorporated into the $SIGNUP mailing.  The Welcome message template,
however, is sent in a separate mailing from the $SIGNUP and can be used in
leu of the Welcome template.  Hope this helps.

--Michael