1)    One who is accustomed to add supplications after the Amidah and is in the process of doing so when the cantor reaches Kaddish or Kedushah, even though he is allowed to respond where he is (as at this point in the Amidah it is permitted to respond to all of the things to which one is allowed to respond during the blessings of the Shema), it is nevertheless better that he should interrupt in the middle of the supplications and take three steps back and respond to Kaddish or Kedushah. The reason for this is that since all of the time that he has not yet moved his feet he is considered like one who is still standing before the king, and since there is no obligation to add these supplications in every prayer, it is better to step away in order to respond to Kaddish and Kedushah. (Mishne Berurah 122)

2)    If he is unable to find the time to take the requisite three steps back, and he has still not said “Yiheyu Leratzon etc.”, (as he follows the opinion that one may add supplications before saying “Yiheyu Leratzon etc.”), ideally he should now say “Yiheyu Leratzon etc.” before responding to Kaddish or Kedushah. (ibid)