re: general 3D modelling
FWIW: I offered something like that before, with MilkShape (a low cost 3D modelling tool), but nobody tried it
(I used it for the spinning poke balls).

----
Exporting it is relatively easy (the format is known enough to preview in LMT etc). Importing is the problem. There are many unknown things in the file format and recreating items from scratch makes the game crash. Also each 3D program does it slightly differently (that's why I picked one, MilkShape)
Even the highly specialized SZS Modifier changes the game data in some extra ways.

I think the world of DLC editing will remain simple tweeks to existing items. There are very few people interested in creating items (which is why I abandoned ItemTool/LMT).