New Custom Field for W-O-Matic

Hi,
Is there any way I can add new Custom Field in the posts fetched by W-O-Matic
Like by default it creates 3 Custom fields i.e.
wpo_campaignid
wpo_feedid
wpo_sourcepermalink

I want to add another Custom field,

Thumb -> first image in the post

Does anybody know where to edit in plugin php file or any other solution.
Thanks