how to add a footer in the articulate storyline 360

you could create a html page with your footer and embed the complete course in an iframe

If you put the info on the Master Slide, it will appear on all of the slides. 

That's not quite a "footer," because the Player buttons would still appear below it. But that would provide a consistent look.

Here is the User Guide article about using Slide Masters: https://community.articulate.com/series/articulate-storyline-360/articles/articulate-storyline-360-user-guide-how-to-use-slide-masters