The Sharingan cannot copy a Kekkei Genkei, Amaterasu is a technique exclusive to the Uchiha. However, if Kakashi really did obtain Obito's chakra, that would mean Kakashi now has Uchiha blood, technically making him an Uchiha. Considering what Kishimoto has done so far, it's not unbelievable that he would grant Kakashi the power of the Amaterasu as well.