FE (or any other tree software that I know of) doesn't have a specific way of handling adoptions. You just need to decide how you want them displayed in you data. You can add them in the usual way then make a note in their biography tab's bio note section that they were adopted with any info on birth parents that you may have. Or you could list them in the bio note section of the adoptive parents. Or you could make their surname at birth "Adopted." Or.... It's up to you.
