It is possible to add a text via the administrative interface, but is't not possible to change the mail completely. This could be usefull to translate those texts.
If your goal is to translate the text into another language, please see the Mailman-i18n list archives: it's coming in the next (?) release.
No, it's not only to translate. Is't to change those mails completely, and to be able to have a completely different welcome/password reminder/... mail for every mailinglist on the system.
This must be possible with a simple hack imho, just change what file to use, instead of template/file use lists/listname/file. But unfortunately I don't know python that well.
Anyone who would like to add this to mailman ?
Regards and many thanks, Adriaan Peeters