While this does not truly answer your question, there are instances of syncretism during the spread of Christianity into the Baltic region. Paulson notes how surviving folk-songs depict St. George, the dragon-slayer, was incorporated into native Estonian worship as a sort of 'wolves' master'. Instead of being a patron Christian saint, this new St. George was a shepherd (or knight who rode with his wolves - Püha Jüri kutsikad), to whom the peasant Estonians give offerings for the protection and survival of their flock. But they did not ask for permanent protection; just in the spring and fall seasons.

However, there are some issues with this, namely: the accuracy of the folk songs, and the whether the native Estonians believed this Estonian St. George as an independent deity or as a form of intercession. For the first one, I can't speak in detail as to their accuracy, but Paulson does state that the forms are alliterative and later transitional. As for the second one, I cannot give a proper answer without a time machine and knowledge of Estonian, but suffice to say, the native Estonians (and Baltic tribes) acknowledged power and strength above all. Henry of Livonia writes how after defeating the Livionians, they were baptized and converted and went on to become of the core native troops of the German crusaders. On the other hand, after the Teutonic defeat in 1241 at the Battle on the Ice, the narrator of the Livonian Rhymed Chronicle stated how the conquered Estonian tribes rebelled and 'washed off the baptism'.

Sources:

• Ivar Paulson, Old Estonian Folk Religion

• The Chronicle of Henry of Livonia

• The Livonian Rhymed Chronicle

From the Western Hemisphere, many local ways of worship in the Caribbean are syncretic in nature; combining elements of ancestral African religions practices & Christianity. Santería, Candomblé, and the various regional sects of Vodou are three Caribbean examples of religious practices from outside the Christian pantheon which incorporate or analogize Christian saints and dogma into their own practices.

Another take on it would be Rastafari, which arose from the Christian pantheon but which supplants Jesus Christ as the most recent incarnation of God (Jah) on Earth with former Ethiopian Emperor Haile Selassie I. Jesus is yet considered an important figure, but one whose era of prominence ended with the rise of Haile Selassie.

Another example is the Washaat (or Dreamer Religion) of the American Northwest in which Christian teachings, symbolism, and liturgy are incorporated into preexisting Native spiritual ways. Other Native American religions incorporating Christianity in similar ways include the Longhouse Religion of the Six Nations and the modern Native American Church.

Sources:

• Stadius, Martin. Dreamers: On the Trail of the Nez Perce. Caldwell, ID: Caxton, 1999. Print.
• Barrett, Leonard E. The Rastafarians. Boston: Beacon Press, 1997. Print.
• Hall, Gwendolyn Midlo. Africans in Colonial Louisiana: The Development of Afro-Creole Culture in the Eighteenth Century. Louisiana State University Press, 1995. Print.
• et al.

According to Bede's Historia ecclesiastica gentis Anglorum, King Rædwald of East Anglia converted to Christianity in the early seventh century, but rather than abandon his previous beliefs, he merely erected a Christian shrine alongside those to his other gods.

Follow-up question: What about the Hebrew God? Was Yahweh adopted into any pantheons?

Edit: Also, how did monotheism arise in Judaic religion? Looking at Wikipedia, it would appear that the people of the area were polytheistic up until the Diaspora.