custom field 'thumb' > Featured Image

I had an old template using custom fields (field meta_key: thumb) to display thumbnails for each post (over 7000 posts). The images on the site are stored using NGG (14500 images) so the meta value of the field is a url to the image.

The new theme actually has a subroutine to use the first image found if there is no Featured Image but it only looks at attached files which NGG images are not.

Now I want to convert the thumbnails to Featured Images – am open to using phpmyadmin on db tables.

I do not want to import the images into the media library if possible

Or alternatively I’d convert the new theme to display the existing thumb custom field – unsure of the code required to do this..