Hi Shiva, Hi Lukas,
Shiva this is a legal requirement so you shouldnt use User Exit or BAdIs... Well it is as your own risks but I dont recommend it 
It is delivered in standard by SAP but as it is very country specific you will need to use a country version of travel expense accounting. it is an add on to be compliant with US specificities...
Please read the following link :
http://help.sap.com/erp_fao_addon10/helpdata/en/4d/59fb3f0afd3ba4e10000000a42189b/content.htm
Hope this helps.
Regards
Mathieu