patching a source outside of %{buildroot}

Hi, I need to patch a source file in the /usr/src of the build environment. In absolute terms, this is in /var/tmp/build-root/openSUSE_Tumbleweed-x86_64/usr/src It seems that %autosetup resp %patch0 work only in %{buildroot}, which is /var/tmp/build-root/openSUSE_Tumbleweed-x86_64/home/abuild/rpmbuild/BUILDROOT/ Any idea? Thanks Axel