The tokens list is controlled by this .js file: webassist/ckeditor/plugins/EmailTokens/dialogs/EmailTokens.js
That will let you add the tokens to the HTML editor so that they can be added to the body. Then you will have to update the replaceUserTokens() function in the file:
/admin_pm/common/mailing_functions.php on line 231.
If you get them added to both places properly, that should take care of it.
This is something we could help with in a premier support ticket if you run into trouble doing it on your own.