Correct/natural way to say "在我的咖啡可以加糖吗？"
It would be more natural to say 「請幫我的咖啡加點糖。」or more directly 「請幫我加點糖。」(if you're pointing at your coffee, it'll be more natural)。 If you say 「在我的咖啡可以加糖吗？」, it sounds like you're asking if it is suitable to add sugar to your coffee. As a native Chinese speaker, this is my advice.
e.g. Could you please add more salt to this dish? Thank you. 
     幫我把這道菜加點鹽，謝謝。