Need to change function module Source Code But, not allowing Even if i have access key

HI,

I am facing one problem,

Ie, I need to change the Standard function module source code.

As per my requirement, the problem is while posting the miro , system is very slow ,After analysis i was found that,

One query is taking lot of time for delaying the operations at MIRO.

I have found below post.

http://scn.sap.com/community/abap/switching-enhancing-and-adapting-standard-programs/blog/2012/10/23/indexing-mseg-abap-dump-due-to-time-out-during-invoice-verification--miro

As per the above. I need to  Insert the Zindex into the Standard Source code. But If i use the Acess key also.

Source code is also still Non editable mode.

Please have a look on below attached images. Import, Export parameters screens i can able to change it.

But under source code data i am not able to change it. This function module is available under the pool. SAPLJ1I4.

Even if i get the Access key also. I am not able to change the source code.

Please give me any advice. for changing the Standard source code. Need valuable suggestions Please.