Given that we have many comic book questions, and several meta questions discussing that comic books are on topic, can we add it to the site's scope officially? What's the process to do that?
There's nothing in the site FAQ about media, so by default I'd say there's no restriction of which media are within the scope of the site.
If you're still in doubt, turn to the
faq tag on Meta, in which you'll find
Which mediums can be the basis of questions?, where the consensus is that all media are on-topic.
It's true that the site FAQ could use an overhaul. You can help by proposing improvements in the old, but still open meta question on the topic.