I've tried this. It kind of works but I don't recommend it. I gave a cologne of mine more citrus top notes and vanilla base. Of course it's difficult to predict since you won't know the formula. I wasn't totally happy with the result just because the original toilette had notes in I didn't like so much, and though they faded a little, they were still there. I also found it difficult to change the balance of the existing perfume, I had to add a lot of oils to it, and even then it didn't take it so well. Of course I'm a noob, some of the more experienced here will be able to give more insight into this.