Perego,

'to'.   

You could improve your language paying attention to scientific monographs and books but published from Cambridge University Press for standard British English or Wiley for Standard Americal English. Of course, there is recommendable native speaking authors. In addition you could use TOEFL for self-testing. There are shown answers. Your self test scores should be 550-600 points. The latter score is accepted as recommendably for level of knowledge for American Universities. So, test yourself.  

Add to and subtract from in English. Chemistry also allows addition at a site.

Hey Luca!

I also would prefer to use "to", because it´s like "adding something to..." but let´s be a little bit more accurate. 

For example, citing Dean Toste´s JACS from 2014 (DOI: 10.1021/ja503974x):

Abstract: "Herein we report the mechanism of oxidative addition of CF3I to Au(I)..."

So, you have the addition of a first component to the metal.

Best regards!

I vote "to". The others seem less accurate or direct.

Thank you for your answers.

"To" is the correct preposition. We say "add to"