Yes, but you are adding it to a liquid amount that should give you 3.6 or 4.5 mg/ml if you do it as described above in this thread. VG may help with overflavoring, since PG is more of a flavor carrier, so maybe it's good that you got your nic shot in VG-- what will your overall percentage be, or rather, what is it now?
The point is, it doesn't matter if the nic you are adding is at 18 mg/ml, if you do it correctly and please read the post above that tells you how to do that again if you are unclear, but you will END UP with 3.6 or 4.5 mg/ml, which is a fairly low nicotine percentage--- the nicotine will be diluted into the flavor.
Please understand how it works, or if you don't, ask more questions until you do.
My point is that IF you do the nic shot at the APPROPRIATE amount for the VOLUME of your 0 nic e-juice, you will end up with that lower percentage, even if you are starting out at 18 mg/ml, because it will get diluted by the MUCH higher volume of 0 nic flavoring.
I hope that makes sense. Once you try the eliquid WITH that flavor, there are going to be things you can do to a) dilute the flavor more if needed and b) add more nicotine as needed to maintain your 3.6 or 4.5 nic percentage, but I don't think you need to worry about that yet. Again, if you DO need to do that, you will need a good e-juice calculator, but it is totally doable.
Best of luck,
Anna