The easiest way would be to get the .src.rpm for the app to which you want to add the file. When you install a .src.rpm, you don't get a working app; you get the source files (possibly with patches) and a spec file to direct rpm in creating an installable binary. The source file is the object of interest here. It's a plain old, ordinary tarball which you can unzip; untar; add the file; tar; and gzip back to the tarball stage.
Then, use the installed .src.rpm to make a new binary rpm with the text file included.
For a thourough read on the ins and outs of rpm, check out the Red Hat documentation. Look for a book called Max-Rpm (you can read it on-line, or download it to study at you leisure). There are sections which give better explainations and good examples of how to do it.