This is going to depend on how the fields are stored in the database by the other plugin. Are they saved on separate database rows like standard postmeta in WP? Or is the value stored as a serialized array in the database?
The tircky part is not getting the data from the other plugin and inserting it into ACF. The trick is putting back into the db in a form that the other plugin can use.
What type of field are you using in ACF? Since the other plugin allows multiple values I’m assuming that you can add more values. Are you using a repeater field in ACF?
