This is a common issue. Whilst there isn't a way to show adoption properly, what I usually do is to add the adopted child in as a normal child, then to add a note explaining the circumstances in the Bio notes section.
If you want to show the adopted child's biological family as well, you can add a blank box as that child's 'partner', name the box something along the lines of 'X's Biological Family From Here', then 'grow' the tree out from that box as normal.
Hope this helps!
