Is the practice of keeping the awareness in the tanden something that was added in China from Taoism? I never saw any reference to it in the pali sutra's for instance the satipatthana sutra on mindfulness.

It is not written about the tanden, because there was no need to write about it. The teaching of energy and breathing is over 4000 years old, and from there the paths of yoga, tai chi developed. The Buddha taught the breathing practice of Anapasachi, exhaling long and inhaling short, which can only be done with the tanden. You can try it for yourself and feel it. If you do not use the tanden, your energy will get high and your breath short. Only in recent times the tanden is actually being mentioned in the different paths.