SRPM's are source rpms. when you install and srpm:
1. a spec file is put into /usr/src/packages/SPECS
2. the source files (which may or may not be compressed) will be put into /usr/src/packages/SOURCES
what you need to do is open up the spec file and make any changes you want. then run an rpm -ba xxx.spec to rebuild the rpm. once this finished, the newly built rpm will be put into /usr/src/packages/RPMS.
one advantage of using a source rpm is that you can use the same srpm on any architecture. essentially, when you run the rpm -ba, you are compiling the code....it is just easier and more organized this way. the srpm can check for dependencies as well as other things instead of just blindly compiling the code. this way, the rpm that is made from the srpm is, in a way, custom built for your system.