I want to say "Can I add sugar in/to milk?" Should I use "in" or "to"?
If you are holding a mug of milk and adding sugar to it for taste then you will use the preposition to
In this case you might also change the sentence slightly to read:
Can I add sugar to **my milk?**
Using the position to in this context makes sense because to is functioning to show a transfer from person to place. (from you to the mug of milk). The use of in or into would only suggest position and lack the idea of transfer or movement.
I hope that helps.
Some help from: The Ins and Outs of Prepositions By Jean Yates Ph.D.