That can be tricky but here is a step by step process.

Installing spaCy with pixi:

pixi add spacy

The tricky part, python -m spacy download equivalent:
This is where it gets nuanced. spaCy models like en_core_web_sm are Python packages, not conda packages.

So there are two approaches:
Use pixi's PyPI support:

pixi add --pypi en-core-web-sm

or add it to your pixi.toml:

[pypi-dependencies]
en-core-web-sm = {url = "https://github.com/explosion/spacy-models/releases/download/en_core_web_sm-3.7.1/en_core_web_sm-3.7.1-py3-none-any.whl"}

Run the download command via pixi shell:

pixi shell
python -m spacy download en_core_web_sm

I recommend going with the use pixi's PyPI support or add it to your pixi.toml.

I hope this was helpful, please mark it as answered if it was. Thank you!