There are really two questions here, I think:
As far as displaying the symbols, Mathematica uses Unicode, so the table at http://unicode.org/charts/PDF/U20A0.pdf is relevant. See the second page for a list of currency symbols outside the 20A0 block. I don't believe Mathematica generally "knows" these represent currency, they are just characters it's prepared to use.
As far a currency units in its external databases, I don't know if it's even possible to run the heuristics by which it deduces units in reverse and get a list of possible inputs. This is surely a moving target.
