I just find it unusual to try and put additional programs and files into a SWM file after it was originally created. The only time I have seen that done (or should I say tried to be done) had ulterior motives. Not something I would promote.
Alas, I have not personally seen a way to alter them directly. However, there are ways to merge SWM files into a WIM one. That can become huge though.
I would say, however, that unless it is a lot of files/programs, it would be much simpler to just save them separately and install them after the initial installation. The initial installation really should only have/contain the basics needed as to start from scratch and avoid problems.