Hello! Since the way you have worded your question, I suppose the person in question on the tree has already a pair of adopted/step/foster parents?
If this is the case, click on the person; on the sidebar, there is a button "Add second parents". This creates a second set of parents whose info you can enter - and if you only want to add a second mother, you can delete the second father afterwards.
To specify the "type" of parents, you can now click on the "More options" button on the sidebar and then "Set parents". This allows you to choose who the person's primary, second and third parents are, and below also whether they are biological, step, foster, adopted, surrogate etc. - however, this is simply "flavor"/info for yourself and potential other viewers and doesn't change how the relationship is depicted on the tree.
Hope this helps!
