How to add class to ul ol tag in redactor by using formattingAdd
The unfortunate answer here is you can't add classes to ul or ol elements using formattingAdd, at least that's the way I interpret the documentation. See: https://imperavi.com/redactor/docs/settings/formatting/#s-formattingadd
It states:

formattingAdd can only be applied to block tags (p, pre, blockquote, div, heading, etc).

I suspect the etc there also encompasses ul and ol elements, even though they are block elements.
The best workaround I can come up with for this is to have a wrapper around the list and apply the class to that wrapper. As this would be fiddly in Redactor alone, I use a matrix field type to add blocks of content, with the option to add a class to the wrapper for that particular block.
This page describes this method of building out content https://nystudio107.com/blog/creating-a-content-builder-in-craft-cms