As we discussed, JointCollection is limited to a maximum of 20 joints as the Kinect for Windows SDK can support a maximum of 20 joints tracks. But if you want to create your own data point (not new joint) for a skeleton joint you can use the following approach where you can first create a skeleton point with respective positions for the x, y, and z axis:

SkeletonPoint position; 
position.X = 1.1f;
position.Y = -1.1f;
position.Z = 1.8f;

Once the position is defined, create a joint point and assign the joint position with the previously created joint point:

Joint joint;
joint.Position = position;
joint.JointType= JointType.Head;
joint.TrackingState = JointTrackingState.Tracked;

In the end assign the joint the value of the respective joint type. As per this example we have assigned the position for the Head joint within collection:

JointCollection collection = new JointCollection();
collection[JointType.Head] = joint;

You can use this approach to set a joint point to a specific location and then use the same joint once the sensor tracks the skeleton.