Defining Custom Source for Receipt Write Off Activity

Receipt is created with Balancing Legal Entity “AUX” and Receipt write off Activity associated with “033” is getting applied to this receipt.The accounting for this write off activity is generated under LE "AUX" rather than the "033" LE which was set up for write off. Refer attached screen shots for details.

So, We are trying to achieve this functionality by creating Custom Source to derive the required BLE.

We have successfully created the custom source, but we are facing some difficulty in linking the newly created custom source against the Journal Line Types. Could you please provide some more details how we can assign the source to JLT,as the JLT form is freezed and it doesn't allow to enter/update anything.