You can change the userdict-<lang>.txt files. Mary TTS server must be restarted after changes. Be aware to use a |char to split the word and SAMPA phonetics. Finally provide your changes as a pull request (without|) for .txt` (see next para.).
Have a look at the delivered words in the marytts-lexicon repos: https://github.com/search?q=org%3Amarytts+lexicon&unscoped_q=lexicon
- Follow folder
modules/<lang>/lexicon - Open
<lang>.txt
E.g.: https://github.com/marytts/marytts-lexicon-de/blob/master/modules/de/lexicon/de.txt
Alternatively, you can try online generators e.g.:
Here is a starting point for SAMPA phonetics:
German SAMPA phonetics: