Unfortunately right now there is no way to do that. What you could do is list the information under the person's bio section.
John Smith's on the tree under his adoptive parents Albert and Jane Smith. In John's bio section
Biological Father: Thomas Black
Biological Mother: Mary Williamson
This could also be reversed with the person listed under their birthparents on the tree and the adoptive parents in the bio section. Another option is to make two separate trees Adoptive and Biological.