I read the follwoing teshuvah:
You should pray in the Ashkenaz nusach. However, some write that you are permitted to add the extra parts of nusach Sefard.
As with any conspicuous change from the custom of a shul, a person davening in a nusach Ashkenaz minyan should not daven the Sefard tachanun (due to the prohibition of lo tisgodedu). This halachah is mentioned in Iggros Moshe (Orach Chaim Vol. 3) concerning the reverse question–somebody who usually davens Ashkenaz and is present in a Sefard minyan. However, the Steipler is quoted (Orchos Rabbeinu Vol. 1, p. 41) as stating that it is permitted to add the 13 middos, because this is considered an addition, and not a change.
My question is – nusach sefard tachanun is actually the same as ashkenaz but with just a bit more before nefilat apayaim, so perhaps this should be considered an addition and not a devioataion and then according to the stiepeler gaon z”l it should be said?
This is what the answer means when it states that some write that one is permitted to add the extra part of nusach sefard.
[The original mentioned the 13 middos, but this is a mistake, because the 13 middos cannot be said without a minyan.]