“On May 22, 1908 Fr. Zerchaninov was appointed the Administrator of the Mission to the Russian Catholics. The decree from the Vatican Secretariat of State appointing him specifically states: ‘Therefore His Holiness commands the aforementioned priest Zerchaninov to observe the laws of the Greek-Slavonic Rite faithfully and in all their integrity, without any admixture from the Latin Rite or any other Rite; he must also see that his subjects, clergy and all other Catholics, do the same.’
Subsequently, this command to observe strictly the Russian Orthodox Church’s rituals and spirituality was confirmed during an audience with Pope Pius X attended by Mlle. Ushakova.
In response to Mlle. Ushakova’s inquiry whether the Russian Catholics should hold firmly to their Russian synodal and Old Ritualist practices, or adapt these to the more ‘latinized’ Galician liturgical forms, Pope Pius replied that the Russian Catholics should adhere to the synodal and Old Rite practices with the now famous response in Latin: ‘nec plus, nec minus, nec aliter’ (no more, no less, no different). This principle continues to be observed by the Russian Catholic communities today.” (“Nec Plus, Nec Minus, Nec Aliter: A Brief History of the Russian Byzantine Catholic Church and the Russian Catholics”, copyright 2005 by Reader Methodios Stadnik. Link.)
Priests coming from the Orthodox Church into the Roman Catholic Church retain their priesthood, but do not necessarily celebrate Mass or other sacraments in an official capacity for the Roman Catholic parishes without permission of the Bishop, but he may continue observing it in his private chapel or residence. Circumstances may vary. Canon 899 of the Code of Canons of the Eastern Churches says that a “cleric of an Eastern non-Catholic Church entering into full communion with the Catholic Church can exercise his own sacred order according to the norms established by the competent authority;” however, it says that “a bishop cannot validly exercise the power of governance except with the consent of the Roman Pontiff, head of the college of bishops.” In cases of emergencies, both Catholic and Orthodox clerics may hear confessions and offer last rites. In any case, whether you are Catholic or Orthodox, you can send your name or the names of individuals in which you wish to be mentioned at one of the celebrations of Divine Liturgy (Mass) and I will add these to the list. These individuals do not have to be Catholic or Orthodox.