~는 김에 (~neun gim-e) – since one is doing this, why not do that
This grammar particle is commonly used in situations where one makes a suggestion based on another’s plans.
You can look at this grammar particle to have the meaning of “while you’re at it” or the above explanation, which I would personally prefer.
In a way, “while you’re/I’m at it” might seem a little pushy in the sense that you’re commanding someone to do something instead of suggesting.
Of course, depending on the context, it can be seen (or mistaken) as commanding as well.
그대는 그대의 아침을 만드는 김에 내 몫도 만들어 줄 수 있을까?
Since you are preparing your breakfast, could you also prepare my share too?
우리 동생은 담배를 사러 마트로 가는 김에 소주를 사 달라고 했어.
Since my younger sibling is going to the mart to buy cigarettes, I told him to buy Soju for me too.