Translation:My friend eats everything except fish.
But it can also mean "in addition to", which is confusing me a lot. Wouldn't that mean, that this could also mean "Everything including fish" instead of "Everything except fish"?
No, it wouldn’t. Krom is a preposition presenting something that is outside the discussed concept. Of course, something outside the discussed concept is something in addition if you look at the whole. Often you refer to this by the words krome and kroma.
Mia amiko manĝas ĉiujn plantojn, krome fiŝaĵon.
Mi friend eats all plants—and outside this vegan concept also fish.
Mi havas edzinon kaj kroman amatinon.
I have got a wife—and outside the concept of marriage a(n additional) lover.