We have been chatting about this (others) here and there, including at the XP9 website concerning transferring a model from Gmax to XP9.
I have heard of several ways, but all require reworking, once its in Plane Maker (PM) or Blender. If you can convert the model to Lightwave Obj format, it will then go into Blender with few issues.
Felix from FFDS.com said the other day that it might be possible to compile to X format (FS9 Gmax SDK using MiddleMan and ModelCommander to auto-generate-save X files during FS model compilations), and take the X file and import to PM.
Havent tried that yet...
But... (big but here).. This means we cannot bring in high density FSX models with close proximity vertices or huge poly count models because we would be using the FS9 SDK Gmax. But this is only for when exporting/importing from Gmax to XP9.. If creating a plane from scratch in PM or Blender, then this info above is of no concern..
Bill