This is a bit beyond the scope of what you’ll probably get help with. If the other plugin uses a custom post type then you can probably add a field to an admin form. Using ACF fields when the other plugin is showing content or doing something with the value in the other plugin would be completely dependent on the other plugin. The best source of information would be the other plugin. If they allow the use of a WP custom field in the same way that you want to use an ACF field then you probably can, but like I said, you’ll need to ask them about it.
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