There is no easy way to do it. Gmax is more for game developers who want to include a way for users to make/modify game content but do not want to develop their own editor.
Read the stickies they have links to pretty much all the free alternatives that can write formats dbpro can use.
There have been plenty of useful discussions about all this in the past. For some reason they all appear to have been deleted. Very disappointing really, this forum is usually very transparent and mature in handling such matters.
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