Hello,
That is currently not possible. It would require developing a complex physics system.

it's really a shame, it would have been fantastic. another question: after realizing a project, is it possible to export a list of pieces that can then be purchased?

Yes:

For public models:
Model page → Inventory (below 3D player)
That's the inventory of your model. If you want to export it to Bricklink, head over to → Bricklink XML
You will get 2 files. One can be updated directly on Bricklink. The other one is the list of part that Mecabricks doesn't know how to convert (e.g. bricklink information has not been filled in yet in the database).

For your own private models, use the url:
http://www.mecabricks.com/inventory/bricklink/{MODEL_ID}.zip
where MODEL_ID is the 11 chars alphacode of your model.

But only with Bricklink?

and about Bricklink, the second file contains pieces of my list??

Thanks