The phrase "excellent addition" means that something or someone is a very good and valuable addition to something else. It implies that this new element has improved an already existing situation, making it better overall.
Note that case studies also make excellent additions to supplemental training material and can be offered as optional choices for learners who want to dig deeper. (elearning.net)
They're also excellent additions to a simmering pot of grains, like rice or barley. (bonappetit.com)
Other excellent additions are thinly sliced onions, chopped cucumbers, parsley — just about any seasonal vegetable. (mixedgreensblog.com)