The purpose of a baby shower is to celebrate the arrival of a new family member. It does not matter if the new arrival is coming via natural birth or adoption, or is an infant or a toddler. A mother adopting a child would be very appreciative to have a shower held for her and the child on this happy occasion.
The planning for such an event is exactly the same as planning any other baby shower. However, if the child is not a newborn, it is helpful to guests to include the size of the child on the invitation for gifts of clothing.