Is there any way to add a pronunciation to OSX text to speech service? I've google for the last few hours without a solution.
Yes, you can affect macOS's text to speech pronunciation. Everything you need to know is in Apple's Speech Synthesis in OS X.
Try these two commands in Terminal.app:
Now to have the individual letters spelt out:
say "[[char LTRL]] cat [[char NORM]]"
See Use Embedded Speech Commands to Fine-Tune Spoken Output for a long list of modifiers.
If you need to speak in another language, be sure to use an appropriate voice.