Storing ABAP Code 
You can write your own ABAP code and assign it to the SAP target fields.
To change ABAP code, you require the relevant developer authorization. For authorization object S_DEVELOP, at least activity 02 and any values for the other authorization fields must be selected.
From the list of steps, choose Maintain Field Mapping and Conversion Rules. A dialog box is displayed.
Position the cursor on the appropriate SAP field.
Choose Insert Rule. A dialog box appears.
Set a flag under ABAP Code. Choose Continue to confirm. You enter the editor mode where you can store your own code.
Note
Note that a maximum of 24 lines can be displayed. To display the other lines, double-click.
Afterwards, choose Check. An error message is issued if your code contains any errors.
Your ABAP code has been stored.
Caution
Conversion rules already assigned will be deleted as soon as you enter your own code in the editor.