Question: Is it permissible for kohanim to bless Jews outside the context of Birkat Kohanim?
Answer: The Talmud (Rosh Hashanah 28b) states: How do I know that a kohen who goes up to the duchen should not say, “Since the Torah granted me permission to bless K’lal Yisrael, I will add a berachah of my own [for example, ‘May G-d , the G-d of your fathers, add to you a thousand times as many as you are now’ (Deuteronomy 1:1)]? Because it is prohibited by the verse, “Lo tosifu – You may not add” (Deuteronomy 4:2).
The Sefat Emet remarks that one may not infer from this Gemara that a kohen is prohibited from greeting other people with the blessing of “Shalom.” Why? Because the Talmud specifically disallows only the addition of an extra blessing to the mitzvah of Birkat Kohanim. The Talmud only speaks about a kohen who “went up to duchen.” Outside of this context, a kohen can bless a Jew as much as he wants.
This reasoning appears to be essential to understanding the prohibition of “lo tosef” in general. If a person made tefillin with five parshiyot or a garment with five pairs of tzitzit but did not wear them, he hasn’t violated “bal tosef.” The violation occurs only during the performance of a mitzvah. As such, there is nothing wrong with a kohen blessing people outside the context of the mitzvah of Birkat Kohanim.