It is always very risky adding a angel to another angel. Though it is possible. If you know how to sex angelfish (Google/Youtube is the easiest way to describe) As there ARE some differences between Angels when not breeding. So I would advise to get a adult Female, as I find they are slightly more placid with other adults. But still this can not be a 100% that it will work. If and when you get the new angelfish, remove the current angelfish from the tank and re-scape the tank as the fish will think it is in a new environment, and introduce the two a the same time into the tank.
Hope that helps.
Westwood