SpineUtils.putRegionIntoSpineSkin() allows to programmatically put LibGDX TextureRegion into Spine animation slot by replacing existing attachment in cloned spine skin. This way you don't have to change SkeletonData so you can use it for multiple spine animations with LibGDX AssetManager.

SpineUtils.java

/**
 * @author Lukasz Zmudziak, @lukz_dev on 2016-01-11.
 */
public class SpineUtils {

    public static Skin cloneSkin(Skeleton skeleton, Skin oldSkin, String newName) {
        Skin newSkin = new Skin(newName);

        Array<Attachment> slotAttachments = new Array<Attachment>();
        Array<String> attachmentNames = new Array<String>();

        // Loop through every slot
        for(Slot slot : skeleton.getSlots()) {

            // Get the slot index
            int slotIndex = skeleton.findSlotIndex(slot.getData().getName());

            // Get attachments and names from this slot
            oldSkin.findAttachmentsForSlot(slotIndex, slotAttachments);
            oldSkin.findNamesForSlot(slotIndex, attachmentNames);

            // For each attachment in this slot, copy it to new skin
            for(int i = 0; i < slotAttachments.size; i++) {
                newSkin.addAttachment(slotIndex, attachmentNames.get(i), slotAttachments.get(i));
            }

            // Clear arrays
            slotAttachments.clear();
            attachmentNames.clear();
        }

        return newSkin;
    }

    /**
     * Makes a copy of default skin and replace attachment with the new one
     * @param spineAnim
     * @param region
     * @param slot name of slot that contains replaced attachment
     */
    public static void putRegionIntoSpineSkin(SpineAnimationComponent spineAnim, TextureRegion region, String slot) {
        // Get old attachment
        RegionAttachment oldAttach = (RegionAttachment)spineAnim.skeleton.findSlot(slot).getAttachment();

        // Prepare new attachment
        RegionAttachment newAttach = new RegionAttachment("Attachment");
        newAttach.setRegion(region);
        newAttach.setWidth(region.getRegionWidth());
        newAttach.setHeight(region.getRegionHeight());
        newAttach.setScaleX(Assets.ASSET_RESOLUTION_SCALE); // This is specific for my code
        newAttach.setScaleY(Assets.ASSET_RESOLUTION_SCALE); // This is specific for my code
        newAttach.setRotation(oldAttach.getRotation());
        newAttach.getColor().set(oldAttach.getColor());
        newAttach.setX(oldAttach.getX());
        newAttach.setY(oldAttach.getY());
        newAttach.updateOffset();

        // Find proper slot
        int slotIndex = spineAnim.skeleton.findSlotIndex(slot);

        // Prepare new skin and replace attachment with the new one
        Skin skin = SpineUtils.cloneSkin(spineAnim.skeleton, spineAnim.skeleton.getData().getDefaultSkin(), "Custom");
        skin.addAttachment(slotIndex, oldAttach.getName(), newAttach);

        // Set up new skin
        spineAnim.skeleton.setSkin(skin);
        spineAnim.skeleton.setSlotsToSetupPose();
    }
}