First, make sure you have a slot to add your new attachment to.
Second, the same tools in Mix and Match still work.
Mix and Match assumes that you pre-define a template attachment in Spine so that you don't have to manually align or rotate your new sprites to the skeleton. What it would choose to replace is the template, not just any attachment.
If you opt to not use Skins it recommends, you just create a new attachment and set the slot's attachment to the new attachment.
void SlotAttachmentFromSprite () {
SkeletonAnimation skeletonAnimation = GetComponent<SkeletonAnimation>();
Skeleton skeleton = skeletonAnimation.skeleton;
Slot slot = skeleton.FindSlot("slot you want");
Sprite yourSprite; // The sprite you want to attach.
Material sourceMaterial; // The material from your skeleton.
var newAttachment = yourSprite.ToRegionAttachmentPMAClone(sourceMaterial);
// Set your own values to align.
newAttachment.X = 0;
newAttachment.Y = 0;
newAttachment.Rotation = 0;
newAttachment.UpdateOffset();
slot.Attachment = newAttachment;
}
But you would need to set the rotation, x and y yourself. And then call UpdateOffset.
With either option, you also still need to set the texture properties of the sprite as mentioned in Mix and Match documentation.
https://github.com/pharan/spine-unity-docs/blob/master/img/spine-runtimes-guide/spine-unity/mixandmatch-texture-inspector-options.png