At the moment, there is no way to directly show adoptions on trees; Family Echo may be working on a solution. However, I have found that a workable solution is to add a genderless (white) box as a partner of the adopted child, write in it's name slot something along the lines of: 'Timothy's biological family from here,' and then grow their biological family's tree from that box. You can also add in explanatory notes in the Bio Notes section.
Hope this helps!
