The four they own, we're fitted with small cargo doors, as opposed to standard ones, to save pennies. Now, in a world of Dreamliners and A350s, The major life of a second hand 77W is as a freighter, but no one wants these birds cause they are not suitable for freight operations. So yes they own them, but no one is going to buy them.

John Borghetti and his team made a lot of stupid decisions during their decade at the helm. This really was the cherry on the top.

Is there no way to retrofit a larger door onto the aircraft?

I can think of two options, at opposite ends of the cost spectrum: a sawsall and duct tape, or send it back to Boeing for modification. One of those options is crazy reckless and the other crazy expensive. I will leave it up to the reader to decide which is which .