Do not diminish anything of the Torah
“Do not diminish from it” Devarim 13:1 applies to both the Written and Oral Torah.
If a wise person says that a commandment from the Rabbis is from the Torah he adds to it, should he take anything away he diminishes the Torah.
The Sages however added commandments of the Rabbis by way of a protective measure or decree for example,
reading the Megllah/ Book of Ester on Purim and lighting the Manorah/ Channuka lamp as praise and thanksgiving to God
They said these are laws of the Rabbis .
This is not under the prohibition of “Do not add to it” since they clearly did not say the Holy One commanded this mitzvah to be added to the Torah.
Similiarly if a Kohen said one extra blessing with the 3 blessings of the Kohanim as written in the Torah, this would be adding.
This applies in all places at all times.