Well. I just realised, that I formated my quesion wrong.
I appreciate your effort to help me and sorry for wasting your time.
After using the layer_sequence_create (which requires layer_id, x, y and sequence) ... after that ... can I change its layer_id afterwards?
The reason I would need this, it's because I use layers for sorting correct depth. I was toying with an idea to use sequences for character animations, different body parts, equipment, and maybe even different characters, all in a single sequence. But if I can't change the layer of a sequence after creating it, then I can cross out this idea.