is there any possibility of adding posts from a backup onto a working instance without replacing everything that’s already there?
The best I can think of: Restore the backup to a temporary instance, then use this guide to export the topics you want and import them on your main install:
The caveat is that the category that the topics are restored in to needs to be a new one. You can move topics around after that.
But if you mean restoring posts into an existing topic, there’s no way that I know of.
Perhaps say more about your particular situation.