I take it you've seen the tutorials here? (There isn't one about the additive blending there, its just mentioned in there)
http://unikronsoftware.com/2dtoolkit/doc/To mix additive & non-additive sprites in the same collection is a slightly advanced feature (and I'm not sure if anyone else has actually used it apart from us). It used to be much more obvious how to do it in the past, but when the interface got revamped it got made a bit more obscure as it wasn't widely used.
Here are the steps to do it -
1. Create a sprite collection as usual and fill it
2. Go to the sprite collection settings overlay, then tick "Premultiplied Alpha"
3. Double click on the Material listed at the bottom of "Settings" and switch the shader to "tk2d/PremulVertexColor" (the default is tk2d/BlendVertexColor)
4. Go back to the sprite collection editor, find the sprite you want to make additive, and tick "Additive" (This is only visible when premultiplied alpha is on).
5. Commit.
That should be it. When you create that sprite, it'll be additive.