I've been using YACReader Library for a bit now, and I've a couple of comics that are not on Comicvine.
Is there a way to manually add tags to these comics? I've been looking for an option to do so, but I can't find one.

Yeah, just click the edit button (the pencil), next to the comic vine button, and edit the fields yourself

Yes, I know about those metadata fields, but I was wondering if there was a free tagging system and a field specifically suited for it? Apologies for not being specific enough.

Additionally, I've noticed that when a comic doesn't have a number (e.g.: A set of unsorted stand-alone comics), the comics are displaying the filename rather than the title that was set in YACReader. Is there a reason for this behaviour?