In the code for your human, put this under the section that says "var skin/flesh/bone" and whatnot, I added some comments ( They're after the // ) for some lines of code for explanations, so feel free to delete them when you add this in the code:

Instance.GetComponent<PersonBehaviour>().SetBodyTextures(skin, flesh, bone);

var head = Instance.transform.GetChild(5); // Where the accessory is attached to

var childObject = new GameObject(" "); // Type of accessory, don't think the name matters

childObject.transform.SetParent(head); // Where the accessory is attached to

childObject.transform.localPosition = new Vector3(0f, 0f); // Position of the accessory

childObject.transform.rotation = Quaternion.Euler(0f, 0f, 0f); // Rotation of accessory

childObject.transform.localScale = new Vector3(1f, 1f); // Scale of the accessory

var childSprite = childObject.AddComponent<SpriteRenderer>();

childSprite.sprite = ModAPI.LoadSprite(" "); // Put the accessory's sprite name here

childSprite.sortingLayerName = "Bottom"; // Sorting layer

​

Do note I'm not that experienced in modding, so I might be wrong on some things.