Rashi says about the altar that we can't wave a sword over it in משפטים, the maftir. Can I add his opinion (above included also) to the list of אלו דברים שאדם אוכל פירותיהם בעולם הזה in Shacharit? What halachot might prevent me from adding to the tefillah? This particular mishnah appears to have several sources, and the version in many siddurim does not match תלמוד בבלי מסכת שבת, קכז עמוד א -ב or משנה מסכת פאה, פרק א, משנה א but I know is discussed here.
If I understand you correctly you want to know about appending a clause into this paragraph because you'd like to include a prayer for family unity.
The minhag is to say this "memra" after Birkas Kohanim so we may follow the b'racha with mikra, mishna, and memra. (Mishna B'rura 47,21). In fact, the statement as it appears in the siddur is a conglomeration of several nusachaos by Amoraim, etc., as you alluded to. In any case, the version canonized in the siddur is a list of mitzvos with a unique quality; namely, they grant a benefit to the doer in this world, beyond the reward they earn the doer in the world to come.
With this understanding in mind, adding to this list would be out of place, 1) because whatever more I add does not in fact share the quality of the other elements in the list; the list already includes all those mitzvos that share this quality, and 2) the list is not one of requests.