How to execute task on composite build which does not exist in some of the composite build projects?

Hi, I am using Gradle 5-rc3. I have a huge composite build of multi project builds which also includes plugins used to build those projects.

Is there an easy way to execute task on composite build which does not exist in some of the composite build projects?

To go a bit further, why is it necessary in the composite build to define task dependencies for tasks which exists in included projects. It would be much more convenient if the composite build behaves like multi project builds and could discover automatically launch tasks when they exists in sub projects.