To be honest, I've only found this word in names of martial arts clubs, and I know too little of martial arts to know how serious these clubs are. Since I don't know how it's written in Japanese, I haven't been able to find any Japanese examples, only English examples. A random sample from the net:
The general perception that a Black Belt is the highest level in martial arts is a misconception. While it is the end of the colored belts, Black Belt is a new beginning. A journey towards the understanding of the Zen Do Ka and yourself. A True Zendoka never ceases their exploration of Martial Arts and will continue to discover more in the Dan Grades, contributing to the art of Zen Do Ka with skills they have personally developed or adapted from elsewhere.
Zen Do Ka. The Japanese word Zen means "meditation" Do means path, Ka is the spirit on the path. A Zendoka is a spirit on the path of enlightenment. Many travel, some finish. The question is "does such destination exist, or is the path the destination?"
To put it blunly, is "zendoka" a proper Japanese word or it just some bullshit word which Western martial arts practitioners have invented? If it's a proper word, is it a word a Japanese native speaker would use?