I’ve found older questions about this, but since this forum has no way to upvote feature requests, I’m opening a new topic for this (which will probably get closed soon, and then on and on and on …)
So, since I’ve already had a huge library of videos once I started to use Infuse on my AppleTV device, it’s way too much work to mark each of them as “watched” manually.
User interface wise, it should be not much work to add a dialog that asks me if I want to mark everything as watched when I add a folder from my NAS, for instance. Or some other way, maybe in the global settings. It’s rarely needed, so even something a bit more hackish would be sufficient, though, sadly, AppleTV has no Terminal.app 
Or if I could simply upload text a file to the app’s local file store that contains the titles or file names I want to mark watched. Sure, transferring that file will be a bit of a hassle, but there are plenty of guides for that. Anything is better than me having to go over 100s of titles and mark them watched one by one!