So a spanish guy named Trifindo released a tool yesterday, which you can use to map for Pokemon DS games.
This thread is pretty messy. I will update this thread somewhen else. Latest version is 1.18.1
Updates:
-----------------------------------------------------------------------------------
Updates v 1.3:
Updates v 1.4:
This version has a new option for importing tiles. Now tiles from sketchup should work fine
Just make sure that the model has only quads and trianlges, or triangulate it when exporting
Updates v 1.5:
-Bug