Now, I would imagine it is possible. I used to add flavoring to flavored ejuice to enhance it. But the thing is, once you go that route, it's just as easy to totally diy your own ejuice.
And flavorings aren't always for Ejuice only. Lorann's oils are made for the baking scene to flavor candies and such, but some of the varieties that don't have alcohol in it make for some good ejuice flavorings. I am a big fan of their Guava flavoring. Capella's flavorings were also designed for the food and beverage industry, but have also made a big scene in the vaping industry. Many flavorings that are big in the industry have other uses, so the flavorings I'm not worried about.. it's the regulating of Nicotine.