Is it permissible to add the word "tranquility" (السكينة) when returning a greeting, so that it becomes: "Wa alaikum assalam wa al-sakinah, wa rahmatullahi wa barakatuh" (And upon you be peace and tranquility, and the mercy of Allah and His blessings)?

It was previously discussed in Fatwa (definition) no. 30741 whether it is permissible to add to "wa barakatuh" (and His blessings) in the salutation. It became clear from the narrations of the Salaf (pious predecessors) that the matter is broad and adding to it is permissible, even though following the Sunnah (definition) is paramount.