First, add me to I,
You may get me back -
But I won't be exactly the same.
Isomer, a compound with the same chemical formula but a different structure
I might be sweet,
Somersweet, a sugar replacement
OP's suggestion: S'more: A s'more (sometimes spelled smore) is a traditional nighttime campfire treat popular in the United States and Canada, consisting of a fire-roasted marshmallow and a layer of chocolate sandwiched between two pieces of graham cracker.
I might be Jewish,
Somers, a jewish name
OP's suggestion - Omer(s): (unit), an ancient unit of measure used in the era of the ancient Temple in Jerusalem AND Counting of the Omer, Jewish ritual during the forty-nine days between Passover and the Feast of Weeks
I might be a city plural.
anagram for the plural of Orem, Utah
OP's suggestion: Rome(s), Italy
You may need to turn around.
Add me (maybe my imposter) to time,
You may lighten up a little.
Add me to a set,
You'll find yourself in various places.
Somerset, a county in England
OP's suggestion: Also places in Australia, Canada, Bermuda, the U.S.