Hallmark works with both freelance and salaried writers and artists. Visit their Web site or check out the Writer's Market book to get more detailed information.
Hallmark employs a large in-house staff of writers, editors and other creative professionals to produce its greeting cards. As you might imagine, there's a lot of competition for such jobs.
I think the best way to find out that is to go to their website and see if you can get more information. You might also want to go into one of their stores and see if they can help you.