I personally think this idea would be a great addon to have! While I know it is currently to manually record windows when a game is not supported, it would be a great addition to allow us to select our game (just like how you can select games to show up as your activity in Discord) and have the same features like any supported game (auto recording full session or manual only).

For example, I like playing my Nintendo Switch games on PC with my capture card using the app from my capture card, the app itself doesn't have any recording options, and even if it does, I would prefer having one app for all (Outplayed). If I'd be able to add this custom app I can just take highlights whenever needed instead of being forced to remind myself of clicking the record button and being forced to record the whole session.