There are already a few answers above, esp., @stonecutter and maybe @jackdonwang as well. One thing about their answers is regarding "可以", as @claw pointed out: it's not common for a native speaker to use 可以 in such scenarios.
Finally, regarding @hsiaoyang's answer. It's too long to add comments below his/her answer to I put them here:
Not sure where @hsiaoyang grew up, but it would be rare for native chinese speakers (including myself) from mainland to say the ways he/she suggested. For us, "請幫我的咖啡加點糖。" is usually said as "请给我的咖啡加点糖" or "请帮我给我的咖啡加点糖" (maybe because it's not semantically right to say "帮咖啡“？). And more importantly, "在我的咖啡可以加糖吗？" seems quite unnatural, or even ungrammatical -- I do not think I have ever heard, or would expect any mainland native speakers speaking that way. Lastly, " 幫我把這道菜加點鹽" also seems not quite natural for us (probably also never heard of); the closest way we say for the intended meaning is "(请)帮我 给 这道菜加点盐". Other parts of @hsiaoyang seem to be fine.