Hi Sunrier,
In QA01 Inspection lot there is one field Short Text. Use this field to maintain batch number and click on long text and use that field to maintain Batch number. You can FM READ_TEXT to fetch the data from long text.
Else refer the below thread which can help you to add custom fields.
http://scn.sap.com/thread/3377677
Thanks & Regards,
Ramagir