"Adding on" means including something extra, like a fee, feature, time, or space. It can also mean extending a building or plan by attaching a new part.
We turn pain into suffering by adding on all kinds of beliefs, interpretations and judgments to it. Brenda Shoshanna
Hereby I send you back the 'Winter' [a painting]. I hope you will be better satisfied now. After alternately smashing away and adding on the cart some new figures, finally this one climbed up, which I hope will do better than its predecessors.. Anton Mauve