Modify Standard Codeunit using Extensions

Hi all,

I want to add fields to Item Tracking Lines using extensions. But for this I need to modify standard codeunit 99000830 Create Reserv. Entry which actually is not possible to be done using extensions. It gives an error saying that the standard codeunit cannot be modified. 

So can anyone suggest is there any other way to achieve this and add fields to Item tracking lines using extensions ?