Custom Field = Featured Image URL

I am currently using Social Deals Engine and Postie to add posts to my website. Postie works great but there are a couple of fields it can’t handle because the custom field names contain brackets [] and it breaks the Postie shortcode. The field I need to populate is the main deal image. Is there a function (maybe in functions.php) I can use to tell the _wps_main_deal_image[src] to use the same image url as the featured image?