Files2rpm allows you to create a RPM package from files contained or installed in the filesystem.
Very important: rpmbuild is required for proper builds. For non-root builds you have to configure your '$HOME/.rpmmacros'. See RPM documentation for that.
Suppose your favorite application 'Foo version 1.2' is already installed in '/opt/foo-1.2/' and you want to create a rpm from it. Just do:
$ files2rpm -N foo -V 1.2 /opt/foo-1.2
These details are provided for information only. No information here is legal advice and should not be used as such.