Can I have a Signature at the bottom of all my marketing emails?

Try this...

Go to marketing/Templates and Add a New template called "Signature". Enter all the nice text and formatting you want at the bottom of your emails, and save it.

Whenever you create content for a new mailing, start by selecting this template and clicking on "Use a template to send" next to it. Then you can "Edit mailing content" as usual and add the body of your email and edit the signature if needed.