There is no written source for this in Poskim, neither the Rishonim or Achronim, and so writes the Chavos Yair 55 “There is no source for requiring Kaddish to be said with approximated feet” nevertheless so is the custom of all Israel dating back many generations, as testified by the Kav Yashar 50 regarding Amen Yihei Shmei Raba “I have received from an elder Chassid that when one recites Amen Yihei Shmei Raba he needs to approximate his feet just like by Shemoneh Esrei.” See Piskeiy teshuvos 56/1 footnote 7; Nitei Gavriel 41/2
 The reason: As we are accustomed to treat Kaddish similar to Shemoneh Esrei in many matters, including in this. See Admur 56/8 that we take the steps back by Sim Shalom similar to Shemoneh Esrei.
 Birkeiy Yosef 55/9 in name of Mahariy Malko 135; Kesher Gudal 8/5; Siddur Beis Oved 50; Shaareiy Teshuvah 56/1; Chesed Lealafim 55/12; Ben Ish Chaiy Vayechi 10; Kaf Hachaim 55/9
Other Poskim: Some Poskim rule there is no need to be careful in this matter as Kaddish is not simailr to Shemoneh Esrei in this aspect. [Shulchan Hatahor 55]
 Ben Ish Chaiy ibid; Kaf Hachaim ibid