For Alice 2, see the following link:
http://www.alice.org/index.php?page=gallery/index
There is information here on creating 3D models for Alice.
If you wish to build an environment out of existing Alice models, the simplest way to do this is to build an Alice world with the desired models, and then save the world. If you wanted, you could then place this new environment world in the Alice2: Required: templateWorlds directory, and this world will show up in the Startup spashscreen with the other templates (grass, dirt, snow, moon, etc.)
To build an environment class such as now exists in the Environment Gallery is a very complex process, requiring access to the original Alice 3ds files, which are not available.
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It is not yet possible to import 3D models into Alice 3.
I hope that this helps.