Hi @dweb
I’m afraid I don’t have the experience with the Formidable Pro plugin, so forgive me if I can’t help you much.
If you want to dynamically add additional choices to a checkbox field, I believe you can use the acf/load_field hook. This page should give you more idea about it: https://www.advancedcustomfields.com/resources/dynamically-populate-a-select-fields-choices/. That page is talking about the select field, but I believe you can modify it for the checkbox field.
If you want, you can also use Javascript to add the additional choices. This page should give you more idea about it: https://www.advancedcustomfields.com/resources/adding-custom-javascript-fields/.
I hope this helps 🙂
