The original Turkish, as well as the contemporary one, uses the following suffix to express additionality.
-de (2) (not stressed)
Meaning: too, also, as well, as well as
It points at the preceding word.
However, even though this is a vowel-harmonising suffix, it is written separately to distinguish it from the “state of the name” suffix “-de” (in/on/at). In spoken language, “-de” expressing “and” is not stressed, “-de” expressing “in/on/at” is stressed.