Joining two pieces of curtain pole together
What I'd do is drill out the too large hole to make it large enough to take a dowel. You'll need to clamp the curtain pole to ensure you get a clean hole.

Then glue a dowel into hole and cut it off flush - use a hacksaw and then sandpaper to finish.

Leave this to dry and then drill a new pilot hole in the dowel to take the joining screw.

If you don't have the tools/expertise for this then using glue and matchsticks/wooden kebab sticks to fill the hole like a rawlplug should work as well. This has the added advantage that you might not need to drill a new pilot hole - just screw into the mass of matchsticks. However this won't be as strong as using dowel.