I can't really respond about the additives question, as I haven't used those (yet), but I do use the Convatec bags and it sounds like they are similar to the ones you use, at least the opening. At first I also had difficulty opening these, but now what I do (and this may be a bit difficult for me to explain) is "roll" it open using thumb and forefinger. Hold the opening flat between your thumb and forefinger, and slide your thumb along in the direction of your forefinger so that the material at the opening slides as well and begins to pull the two sides apart as the material curls. I wish I could think of a good analogy, but I'll try to explain better if you would like, just let me know. Hope this helps.