I think one method is keep one code base, when export, manually add or not add these additional files per release requirement. But it require you to remember doing this each time export a copy, which is easy to forget.
I created a branch for the project in SVN with these files included, but problem is each time i change the trunk, i need to do it again in the branch, or a merge. How can i get desired release without needs to maintain two code base? Or is there a way to auto sync specified part files of two projects (trunk,branch)? Any suggestion?
I would just keep all the files in a single branch, and tell my release mechanism which files should be included. Without knowing how your release process works, it's hard to say more...