It really depends on the theme and how those custom fields are stored.
But basically you need to create your new fields and use different names that the other plugin uses. Then you need to create and acf/save_post action https://www.advancedcustomfields.com/resources/acf-save_post/ that gets the values from your new fields and stores them it the fields used by the theme in the format that the theme needs them to be in.
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