The code I posted above will get the value stored by the other plugin. In order to use ACF to store a value that the other plugin can use, you will need to create an acf/save_post action.
http://www.advancedcustomfields.com/resources/acfsave_post/
In this action you’ll need to get the values stored by ACF, convert them to an array and then use updte_post_meta to put the value into the database.
http://codex.wordpress.org/Function_Reference/update_post_meta
