If you are not adding the SO2 and the PPM is below 10 in your finished product you do not need to declare it. If you are putting "NATURAL" on your label be vary wary in any SO2 you have in the product, even if you declare it. The natural claim has been coming under fire the past 10 years or so in the US even though there is no regulatory definition beyond natural flavors, and chicken (thanks USDA).
You can declare it by two options:
1. As the last ingredient on your ingredient declaration.
2. Separate statement, similar to allergens, such as "Contains Sulfites".
"Treated with Sulfites" or something of the like is not appropriate as it is not clear if sulfites levels are below or above 10 PPM...consumer confusion, which you could get called out for. However, I see this all the time on labels so not really sure how much enforcement there is on it.
If I were you I'd fix the issue with your suppliers and reject any raisins incoming that have sulfites at 10 PPM or above.