우유 좀 넣어 주세요. is right. Or you can also say 우유 조금만 넣어주세요. to show that you really want someone to put "a little!" milk.

우유 좀 넣어 주세요. Please put in milk a little bit more 우유 조금만 넣어 주세요. Please put in a little milk i think if u said first sentence in cafe, the clerk understand you need more milk in your coffee if you want put in the milk a little, u should say like a second sentence i hope my grammar isn't wrong!! :)