In Dask's Helm chart, there are variables EXTRA_PIP_PACKAGES and EXTRA_CONDA_PACKAGES:
https://github.com/dask/helm-chart/blob/master/instructions.md
Is there any way to use my already existing requirements.txt there?
No, but that's a fine suggestion and would be easy to implement.
In practice we find that people use EXTRA_PIP_PACKAGES for only slight modification of their software environment. When they want to make larger changes they tend to just make new docker images themselves (though obviously this isn't for everyone)
You may want to look at Dask's default docker images here at https://github.com/dask/dask-docker