Adding to accented characters available on OS X Keyboard If you hold down a letter then OS X will show you options for accented versions of that character.  This is very useful when typing in other languages.  

However, not all the versions of that letter are always available.  For example, if I hold down the [S], then I can choose ß, ś, or š.  However, I'd like for it to also display the turkish ş. Is there a way to add that character to the accent menu?

I'm looking for an easy way to type Turkish characters on my english keyboard.
You can find the answer where this question was asked back in 2011:

How to add characters to the press and hold character picker in OS X Lion?

An extra step is needed for El Capitan:

The keyboard&#39;s plists of PressAndHold also can no longer be modified to add the new accents in El Capitain