@kadak Sorry for the late reply, just came back from a short vacation. Glad you've figured it out.
The main UnityEditor import procedure is located in
ImportSpineContent(). Nevertheless, you don't have the Unity Editor functionality available for asset creation and manipulation, so you need to resort to different means at runtime.
kadak It worked, the only thing to keep in mind is when loading the textures from disk you have to assign the same name they had from the spine export, otherwise they do not match the ones in the atlas and it doe not work
I'm not sure of how exactly you are assigning Textures and creating the AtlasAsset, but the names of the atlas page texture files are referenced in the
.atlas.txt file, so these have to match.