I'll put it in math terms, because it's really the only way I understand. Hope it helps.
In addition, or in addition to = x (idea) + y (in addition, or in addition to)
For example: I think that birds are so high, and it's awesome (x, idea). In addition (y), I think birds look super cool also.
You need an idea THEN you can piggy back off the idea with "in addition" (as long as it's relevant).
Addition is literally the action or process of adding something to something else. This varies depending on the context.
For example: The house became bigger with the addition of more rooms.