Right now we only add official slackware packages and those in SBO.
Still, it looks like an interesting package, we will try to create it or create a docker containing the tool with access to the devices /dev part of unraid.
As @Kilrah said there are several applications in the app store, see if you need any of the current ones and if not then create a docker only with ffmpeg.
It is true that there are several, but they all have something else, really with only contain ffmpeg there is none and the option of dockerhub if you are a new user can be more complicated. But you are right and there are several. Thank you.
Best regards.