Seems Like A Bit Of An Odd Thing To Believe...,,,for a self professed existentialist...but hey - another of my groups is 'I am full of contradictions' so I guess that's ok then.
I believe Karma is actually an internal filter we carry with us and apply to random chance - a way to make meaning from life's startling randomness.
A means to try and apply some sense of justice to an unfair world.
However in the Buddhist tradition the word really means action or deed -the creation of our next life by the actions in our current one...so if you ignore the bit about reincarnaton - in essence it's kind of existentialist anyway...
Existence precedes Essence.
There is no fate but that we make.