Dupont: yes, you can do that. But as you pointed out, you aren't going to know the brand, strength, or mixture of the recipe. Whatever you add will alter it quite a bit, I'm afraid. Some vendors (the better ones) use multiple flavors and brands to concoct their juices.
The best thing is to purchase the same thing in zero mg and mix with your current bottle. Otherwise it's going to dilute it a lot.
Sent from the beach