Although this is a more speculative question than ones based on facts, it may be possible to give a reasonable (logical) answer.
First of all, St. Seraphim of Sarov (1754-1833) was glorified (canonized) by the Russian Orthodox Church on July 19, 1903. What is interesting here is that Pope St. John Paul II referred to him as a "saint" in his book Crossing the Threshold of Hope (page 17) - Seraphim of Sarov (Wikipedia)'
In Pope John Paul II's apostolic letter Tertio Millennio Adveniente (1994), he made a strong allusion to Martyrs of Christ of various faiths: Catholics, Orthodox, Anglicans and Protestants:
37."The Church of the first millennium was born of the blood of the martyrs: "Sanguis martyrum - semen christianorum".(21) The historical events linked to the figure of Constantine the Great could never have ensured the development of the Church as it occurred during the first millennium if it had not been for the seeds sown by the martyrs and the heritage of sanctity which marked the first Christian generations. At the end of the second millennium, the Church had once again become a Church of martyrs. The persecutions of believers — priests, religious and laity — has caused a great sowing of martyrdom in different parts of the world. The witness to Christ borne even to the shedding of blood has become a common inheritance of Catholics, Orthodox, Anglicans and Protestants, as Pope Paul VI pointed out in his Homily for the Canonization of the Ugandan Martyrs.(22)
This witness must not be forgotten. The Church of the first centuries, although facing considerable organizational difficulties, took care to write down in special martyrologies the witness of the martyrs. These martyrologies have been constantly updated through the centuries, and the register of the saints and the blessed bears the names not only of those who have shed their blood for Christ but also of teachers of the faith, missionaries, confessors, bishops, priests, virgins, married couples, widows and children.
In our own century the martyrs have returned, many of them nameless, "unknown soldiers" as it were of God's great cause. As far as possible, their witness should not be lost to the Church. As was recommended in the Consistory, the local Churches should do everything possible to ensure that the memory of those who have suffered martyrdom should be safeguarded, gathering the necessary documentation. This gesture cannot fail to have an ecumenical character and expression. Perhaps the most convincing form of ecumenism is the ecumenism of the saints and of the martyrs. The communio sanctorum speaks louder than the things which divide us. The martyrologium of the first centuries was the basis of the veneration of the Saints. By proclaiming and venerating the holiness of her sons and daughters, the Church gave supreme honour to God himself; in the martyrs she venerated Christ, who was at the origin of their martyrdom and of their holiness. In later times there developed the practice of canonization, a practice which still continues in the Catholic Church and in the Orthodox Churches. In recent years the number of canonizations and beatifications has increased. These show the vitality of the local Churches, which are much more numerous today than in the first centuries and in the first millennium. The greatest homage which all the Churches can give to Christ on the threshold of the third millennium will be to manifest the Redeemer's all-powerful presence through the fruits of faith, hope and charity present in men and women of many different tongues and races who have followed Christ in the various forms of the Christian vocation.
It will be the task of the Apostolic See, in preparation for the Year 2000, to update the martyrologies for the universal Church, paying careful attention to the holiness of those who in our own time lived fully by the truth of Christ. In particular, there is a need to foster the recognition of the heroic virtues of men and women who have lived their Christian vocation in marriage. Precisely because we are convinced of the abundant fruits of holiness in the married state, we need to find the most appropriate means for discerning them and proposing them to the whole Church as a model and encouragement for other Christian spouses."
The real crux to this question rests in the possibility of Dietrich Bonhoeffer being declared a martyr officially by the Catholic Church. Obviously he could not be neither beatified nor canonized as a martyr of the faith, since he was not of the Catholic faith. That said, the possibility of him being beatified and then canonized as "martyr of Christ" could be done, if one could find a Catholic bishop to commence the process of canonization at the diocesan level and follow through at all the other levels, just like any Catholic raised to this honor. Rome would respond accordingly.
The Catholic Church has many martyrs who shed their blood under the Nazis!