Here is a way to add a unconnected person to a AMT.
Pick a person (i.e. yourself). Add a Relative as a brother/sister. Then open the profile of this new person, click on "edit this person" Then on the Relationship Tab I removed Both Parents. You can also rename the person at this point and it will be a Isolated Person, unconnected to any other in your tree, but still appear in the People List.