Hi @charlie.brooker,
With the latest version of honeybee-radiance-postprocess you can use the CLI command,
honeybee-radiance-postprocess translate annual-daylight-npy-to-ill
Add --help to the end of the command to get the help message. If you did not rename the results folder you can run it as honeybee-radiance-postprocess translate annual-daylight-npy-to-ill results.
If you want to use it in Grasshopper you can wrap it in a custom component. See for example HB Annual Daylight Metrics for how to call commands from honeybee-radiance-postprocess.