I have added Lorann's to premade juice to punch up a flavour. I wouldn't add it to the entire 30 ml bottle though...do you have any smaller bottles you could use to make a few mixes with? If not, grab a few small plastic or glass dropper bottles (3ml or 6 ml) and use them to mix in. If you add to the whole 30ml bottle, you risk ruining the whole batch of juice. The hobby lobby may have some dropper bottles?
What flavour do you want to add to the mocha latte juice? More coffee, a creamier flavour, sweeter, etc? For Loranns, Keoke Coffee tends to be a better one than the plain coffee, marshmallow adds sweetness, bavarian cream or vanilla butternut are pretty nice as well. You may not agree though.
If you get the smaller bottles, fill it up about 2/3 of the way with the premade juice, then add a drop of flavouring and test it. Add a drop at a time til you get the flavour you want. I find with Loranns it is easy to overload, and you'll know when you do because there will be an alcohol/chemical type taste.