Hi Pavel,
At the moment we are planning to port the plugin for other applications (Maya, etc).
Nonetheless, in the future we'll try to develop some features for game developers...
Try to use the example of oak tree, this mesh model contains around 200000 polygons. You can try to optimize it. But I think that to creation the tree with 10000 polygons, you need to create branches as a billboards with the textures of groups branches and leaves.
Yes of course, you can use the "Instanced geometry" mesh builder, and there you can add any own geometry.
Cheers,
Eduard