Brandy Alexander substitue?!
The above answer is correct in that substituting ingredients obviously changes your drink. However, Kahlua is no substitute for Creme de Cacao. Creme de Cacao is chocolate flavored and Kahlua is coffee flavored. Do you have any chocolate syrup? Add just a touch. No, it's not traditional, but there needs to be a hint of chocolate to the drink.
You can, but the drink you're making won't be an alexander. Kahlua can be substituted for creme de cacao. Baily's has a much different flavor. A splash of coffee might make it closer.