You can actually reference a Meta Box Custom Field in WP Frontend User Pro.
First, in Meta Box, get the ID of the custom field.
Then, in WP Frontend User, go under Post Forms and edit the Form you want to use. Add a custom field to the form and take the ID from the Meta Box custom field and put it in the Meta Key field. Your new custom field in WP Frontend User will now save into the custom field you created in Meta Box. I hope this helps.
