A more formal way to say something along the lines of "later added."?

all depends on context:

later contended that..

later supplemented...

further acknowledged that...

The best formal way to indicate an addition is the word ADDENDUM.

This is Latin for addition, and is commonly used in such a situation.

Either use a colon following the word and then put in your addition.

Or type out the word and then, below it, start your addition.