If you are talking about an iPad/iPhone/iPod, there is no way to add fonts. But there may not be any need to. To input special characters you can use an app like Unicode Map.
If you are talking about a laptop/desktop running OS X, I think all the characters you need are already in the fonts provided. To have the maximum diacritical marks, go to system preferences/language & text/input sources and check the box for US Extended. Then select US Extended from the "flag" menu at the top right of the screen. Here is a list of shortcuts for making the various diacritics. Just ask if any questions.