I’m importing some data, and generating posts with ACF information, which works well for text fields, and choice fields where the option already exists – but I’d like to add new options to choice fields as required.

I’d hope this is something easy to do, but starting to suspect it’s either difficult, or I’m barking up the wrong tree.

To put it another way, when looking at a choice field in the admin panel, we can type in a list of choices – I want to be able to add a choice to this list programatically

Enter each choice on a new line. For more control, you may specify
both a value and label like this: red : Red

Choices

[Input text box here]

Enter each choice on a new line. For more control, you may specify
both a value and label like this: red : Red