A nice example can be found here: https://books.google.co.cr/books?id=rBiGxrgFk-kC&pg=PA204&lpg=PA204&dq=SFA+model+R+plot+stochatistic+frontier+line&source=bl&ots=tPRgEAtYcy&sig=ACfU3U0uSxgDIkGiMnqS_M9uZE_CCVr7FQ&hl=es-419&sa=X&ved=2ahUKEwjo59PlnZnnAhVN11kKHRfpDeIQ6AEwC3oECAoQAQ#v=onepage&q=SFA%20model%20R%20plot%20stochatistic%20frontier%20line&f=false
Actually, you just need to fetch the fitted values from the regression object.