Royal Edition is basically just a package that includes all DLC including "the royal pack." You can get all the DLC separately that comes with the Royal Edition. That's what I did. I got all my DLC piecemeal as it came out though. So I'm not sure if you can get it as one big package or not. But you can get it without buying the entire game again.
You can add the DLC whenever you want. There is a DLC that gives you an alternate Chapter... I forget if it's 12 or 13. But even that you can go back and do later. So really, add the DLC ad early or as late as you want.