get the bone position of the skeleton character and attach something to that bone position

Bones are also GameObjects. You can use transform.Find() to go down the path of the bone hierarchy and attach your weapon/object by setting it's parent. After parenting you might want to set the localPosition to Vector3.zero and localRotation to Quaternion.identity to align and move the weapon/object to the bone. In such cases it's common practise to add a weapon bone to the rig and attach the weapon to this bone.