The answer to your question is ... kinda.
When you add unflavored nicotine to a juice, you're changing a few of it's characteristics.
First, you diluting the flavor. If the 0mg juice, was for example 20% flavoring and you just added 5% more volume (the nic juice), the flavor has been diluted to less than 20%. Depending on the amount of nic added and the particular flavors in the juice the impact of this can be very little, or significantly change the taste of the juice.
Second, when nic is added like that, it's not just nicotine being added but also whatever solvent the nicotine is in (typically PG or VG). The lower the concentration of the nicotine solution, the more PG/VG is being added to the original juice. In all cases, the PG/VG ratio of the original juice end up getting changed. Once again, this may or may not significantly change the taste of the juice depending on it's sensitivity to PG/VG ratios.
Finally, nicotine can have a peppery taste that can vary widely depending on the source of the nicotine. If the nicotine your shop is using is different than the nicotine the original juice vendor uses, that can again change the taste of the mixed juice.