You can do that but you would need 2.2ml of 100mg/ml nic to get your 10ml bottle to 18 ml/mg which would dilute the flavoring by 22%.
Which could be problematic, or not. If you have any VERY strong flavors, diluting them such might make them more suitable to your palate. You'll just have to experiment to see, but of course don't use the whole bottle of flavored juice, take some off and add some of the +nic juice using the following formula : V1C1=V2C2
So if you want to mix your 0 nic flavor with your 18 mg/mL nic flavor (hopefully same or at least compatible flavors!), in equal proportions it would look like this: the new concentration, C2=V1C1/V2 = (5mL*18mg/mL)/V2 and V2 would be 10mL TOTAL end volume (5 mL of each the 0 nic and 18 mg/mL juice) so C2=5*18/10=18/2=9 mg/mL.
If you need your nic concentration (C) higher than 9 to not give you withdrawal or something, you could do a 2:1 mix with the following result: C2=10*18/V2 where the final resulting volume, V2 is 15mL (5 of the 0 nic juice, 10 of the 18 mg/mL juice) so C2=10*18/15 = 36/3=12mg/mL concentration of nicotine in the resulting mix.
Still too low: try a 5:1 mix -> C2=(10mL*18mg/mL)/12 mL = 15 mg/mL final nic concentration. Getting closer to your standard of 18mg/mL, and note the closer you try to get to that benchmark, the faster you use up and have to reorder your 18mg juice and the longer you still have the 0 nic juice around.
My suggestion, do the 50:50 mixes, vape the 18mg/mL unadulterated juice most of the time if you need to, and occasionally switch to the lower nic mix you made. OR, see if you can wean yourself down to 9 with a natural progression, if this is your goal anyway, using the above examples; making 15mg/mL juice for a few weeks, then 12, then 9, and so on!
As stated above, if you are mixing the SAME flavors, you should end up with a fairly similar, though slightly different end mixture that you will LIKELY find acceptable. If you are mixing two different flavors, good luck, I have had VERY mixed results, good and very, very bad, with this.
Happy mixing!
Carbon