When a child has been adopted he/she has two sets of parents, the biological set and the adoptive set. It is very easy to enter two sets of parents in Legacy and this will allow you to follow both lines. This is especially important now with DNA evidence leading adoptees to their biological families.
In the Family View, the second icon from the left under the person you are working with is the Parents' icon which will bring up the Parents List.
You will do everything else on this screen.
You can see that you can add new parents here and you can also link to existing parents if they are already in your database. After you add your new found biological parents this is what you will see.
Now that you have your unknown parents you can click the Add Father and Add Mother to enter the information you have about them. Now you have this.
You only have one more thing to do. You need to designate which set of parents are the adoptive parents.
Notice that I can mark the relationship as Private or Invisible. I could highlight Expresso Coffee and Peeka Book and change their status to Biological but leaving the field blank is the default for biological parents. It is up to you if you want to designate them this way. To change which set of parents is preferred, highlight the set you want to show as preferred and then click the Select button at the top right. The preferred parents will automatically display when you are navigating in the Family View. If you want to switch to the other set of parents, hover to the side of the parents names and a bar will appear. Click that bar and then you can rotate through the parents.