Where to find R3 Backend Methods?

Hi,

I have created a custom B2B Application from the How_To_Create_Custom_App_ECO5200 Guide. I need to extend the SD Order but we are working with an ERP backend and I cant find the performCustExitBeforeR3Call Method described in the ISA Development Guide. I mostly find CRM methods starting with crm*

Actually I am wondering on how to edit the standard java source anyway because all I see are the referenced class Files in the home/shr/ext project.

Any suggestions? Thanks in advance

edit: The SHRJAV SDA didnt contain any sources thats why I referenced it as a required / non developed component.