Is there any way to change the center of an x3d object?
Using sprite-sheet-to-x3d the resulting x3d file places the pivot in the center of the object (because it was not found in the initial image). It should be on the character’s feet.
I looked into the API and the x3d file but without much luck. Even in the two forums no useful suggestions.
A workaround could be to add half of the character height to the target point in Y but it doesn’t seem like a great solution 
Any suggestions?
You can move TCastleScene. Like MyScene.Translation := Vector3(1, 2, 3). See the manual about scenes https://castle-engine.io/manual_scene.php .
You can also use TTransformNode in X3D. Example of using TTransformNode is also on the above page, https://castle-engine.io/manual_scene.php , in later section “Building and editing the scene”.
https://castle-engine.io/manual_transformation_hierarchy.php has a more in-depth overview of our “transformation hierarchy”.
IOW, the generated by sprite-sheet-to-x3d could wrap things in additional TTransformNode that makes the necessary move.
But note that it has to be configurable, as it will not be suitable for all sprites.