Pass default values in Inbound IDOC through ALE

Hi ,

I have set up ALE connection from my HR system to FI system to transfer employee master data.

But I do want to send Default values in some of the IDOC fields I have employee residence address maintained in HR system but when it transferred into FI system through HRMD_A IDOC type, I would like to set the address to my company address which is constant.

So I am thinking to have a custom IDOC processing code in WE20 at FI system which would set the respective IDOC fields into Default values before FI system gets updated with Employee Details.

How do i set up Custom IDOC processing code in WE20?

And also.

Is there any better way to achive this requirement ?