I worked with Chuck Jodry  and John Walton on an article to showcase how to convert 3DSMax model content so it can be used with GMax.

It turns out this is hard and may be a limiting factor for freeware model developers.

The article is here an I hope it is helpful. The DC-3 model that goes along with the article can be found here.

Edit:The historical discussion behind this conversion project can be found here.