Is it possible to merge video clips before or while exporting? Or is a separate program need?
John
Nope, that's still one for another program at the moment.
As it may not make the final release, might I suggest adding it as a suggestion on the Feature Request forum http://feedback.photoshop.com/photoshop_family where it can easily be tracked? If you post the link to your request here, others can also vote on your request.