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's plists of PressAndHold also can no longer be modified to add the new accents in El Capitain
If your primary purpose is to type Turkish without adding one of the Turkish keyboards provided by Apple, then this can be done by Adding the ABC - Extended keyboard-map. All Turkish letters are easy to type with the "option" key, add this keyboard map, and view the layout using Show Keyboard Viewer in the keyboard (change keyboard icon) menu.
Specifically - I was not sure whether "İ/ı" are available on this layout, but after some experimentation - they are there! "option-w" - is adding a 'dot above' a letter, however when pressing "i" - it actually removes the dots (there's well thought out design for y'all!). [note that specifically ı/İ - are absent on the "long press" mode you're referring to, but using the option key to type is much faster, I regularly type Polish, Swedish, Finnish and now Turkish this way, without even having to switch layouts, and I find it easier than the respective maps] [ Adding letters to the long press mode - is possible as mentioned in in @Tom Gewecke's reply]
I'm using MacOS 10.14, and generally use the ABC - Extended for several languages. Gladly it also works easily for typing Turkish as well (which I've just started to learn, and use to type in Chat windows).
The current mapping is, before the respective letter press:
ç ş Ç Ş : option-cğ Ğ : option-bı İ : option-wö ü Ö Ü : option-uâ î û Â Î Û : option-6
Note that it's possible to press the same "option-shift-(something)" after the letter, but the result is different - the first gives a "combined" letter, while the latter gives a "letter+combining character": for example "ğ / ğ" - are different (might be rendered a bit differently, the latter is "g + combing char brev ˘", in case of "ı" - the latter option might render i with an extra combined dot: i̛ - which is probably not what you want.