Bulk generate thumbnails/featured image

Hello,

I am migrating existing (non-wordpress) data to wordpress. I managed to Import all articles as posts.
Now I need to (1 time action) bulk generate and set the featured images.
This is my situation:

– Post are imported
– as metadata of that post i have set the filename of the image in a field called “backdrop”, (“dummyfilename.jpg”).
– If posts don’t have an image assigned to them the “backdrop” metadata key doesnt exist.

I can’t seem to figure this one out. Any help would be greatly appreciated.