Hi @awc
This is a tricky one which ACF is not yet setup to do.
What you are asking for, if I have read correctly, is the ability to create a select field which you can also save new choices to.
This is currently possible via the radio field, but not yet for the select field.
It may be possible to customize the select2 JS args and allow a custom value to be saved, however, you will then need to append this value to the options page (I’m assuming you have a textarea field with all user choices)?
Checkout the JS customization here:
http://www.advancedcustomfields.com/resources/adding-custom-javascript-fields/
And the update_value filter here:
http://www.advancedcustomfields.com/resources/filters/acfupdate_value/
