[arch-general] How to "decorate" a package build?

Hello, is it possible to decorate a package build, i.e. set some prconditions (like exporting variables) and probably even change the build process (compile instead of copying just the binaries)? E.g., to modify the firefox package update to first compile with "--enable-alsa". Of course, I would have to create my own package in a private repository. The new PKGBUILD file would need to refer the original one, to get e.g. the package version number. However, the private package would always have the same version, so the official package change needs to be hooked to the generic private package: Official Package Changed --«Signal»--> Private Package Update --«Inherits»--> Official Package Update Kind regards P.