It most certainly is! However, the manifest.xml file is actually where dependencies are specified, not in the CMakeLists.txt file. rosbuild only pulls in the <export> section of manifest.xml files for packages that you list a dependency on (or dependencies of your dependencies).
Your package should automatically have the correct includes for Eigen if you edit your manifest.xml file and add a line like <depend package="eigen" />. This manifest.xml line would make the stuff you did in your CMakeLists to find and include Eigen unnecessary.