Restore xml and overwrite existing?

I’m trying to use an external editor to edit the xml export file (change domains, paths) and would like to re-import that same file. I have external sites using post id’s (not post name permalinks) and pages/plugins referencing pages using their id’s. So deleting existing posts and re-importing them is not going to work, since the id’s will change.

Is there a way to change 3.5.1 so that it will blindly overwrite everything instead of considering existing posts and leaving them? I can’t find an option in-app for it, so I’m guessing there might be a php hack that needs to be done.