Amend “Syndication feeds show the most recent” in Admin/Settings/Reading. There isn’t an “all” setting but you can at least increase the number of posts in your feed.
Is there any way at all to create a separate RSS feed that includes all posts?
I’m not sure you’d really want to. You have to provide some limit to the search, in any event, but more than that, news ages. If you only show the first 10 or 30 posts in a feed, then usually by then a reader is going to head over to your site after reading/listening to #30 and see what else you have or … they’re not. 🙂
What you want is to increase the amount of posts that are displayed when someone goes to sign his feed?
If so I believe most of wordpress are 10, 10 but appear as new does not mean that the oldest will not be taken, they are there only in the feed do not appear as unread i think.
RSS feeds themselves don’t monitor read/unread count, that would be your reader program that does that.
Any time you subscribe to a new feed, you get the 10 most recent posts (or however many the blog is set to show you, 10 is default for most websites, not just WordPress).
