On some editions of Talmud Bavli, there is a section entitled, "Musaf Rashi." When were these additions to his commentary found, and are there any references to it in Tosafos?
Mosef Rashi is a modern invention from printers who collect Rashi's writing from other Masechtos which would add insight to the sugya at hand.
As such they are Rashi's own words (or Rashbam or the like when quoting from certain masechtos) but not any new found work of his. It's the same old Rashi. So if Tosafos have anything to say about, it would usually be on the folio the are gleaned from.
Other printers have made similar works called Likutei Rashi.