Fine-tune/adding of new speaker to existing models tts/wavernn

Is it possible for existing pre-trained models tts and wavernn to add a new dataset and a new speaker. and retrain the model for this speaker? if so, how?

Possible. You can use the universal vocoder model released here

To finetune the TTS model for your speaker this might help:

Basically, you need to format your dataset as LJSpeech and resume the training of one of the released models. Good luck.