If you know the Unicode code point for a special character you need, you can type that character using an input method called “Unicode Hex Input.” Activate the Unicode Hex Input as you would any other input. You can find it at the bottom of the language list under “Others” (see image).
To type a character with the Unicode Hex Input, hold down the Option key while typing the 4 alphanumerics that make up the main bit of the code point. Note that the 4 alphanumerics are not case-sensitive (i.e. you can type the letters in lowercase). And because they are hexadecimal, that means all “ovals” are zeroes (not the letter O) and “lines” are ones (not the letter l); the only letters are A to F.
One place where you might find Unicode code points is the GreekKeys 2015 manual, where they give the example of the “capital jot” at code point U+037f. To produce this, I hold down the option key, while typing “037f”. If you have a font installed that include this character, you should see it here: Ϳ.