Inserting text in an action

It sounds like you want the text to be included just in the email, right? Not actually added to the source draft?

If so, you would modify your Mail action step’s body template. The default Mail action uses the [[body]] tag to insert all but the first line of the draft, but a templates can be any combination of static text and tags, so you can edit the action, go to Steps > Mail and change to body template to something like:

[[body]]

My additional footer text

For more details on templates and available tags, see: