Thanks for your question.  This is a really interesting subject and doesn't have a clear, easy answer.  You can add  sulfur to raise the pH, but it is not a quick, easy process and remember that you would ideally need to raise the pH not only in the planting hole but in the range of the mature root zone of the tree for ideal results.  

I don't mean to say that you shouldn't plant a magnolia, there are many beautiful specimens in the area and yours may thrive as well.  I know you want to do the best for your tree so here is a link with more specific information about soil acidification.  https://www.canr.msu.edu/news/reducing_soil_ph_in_landscapes  

Best of luck!