How to extend inside door handle shaft/sprig?

[Graniteguy]

So I thought I was being clever with my custom door panels, and they turned out nice. BUT, they are thicker than the stock cardboard. Close to 1/2 inch thick. Problem is there isn't enough shaft poking through for the door handle to grab on to. I need a good 3/8 to 1/2 inch. It looks like I need another head welded onto the existing one. That doesn't even look possible. Does anyone have any ideas? I'm kinda at a loss here.

Thanks

Brandon from MN