How to add one more field in infotype and PA 30 screen

Hi to all

My requirement is to add a new field in infotype and that should be reflected in PA30 screen.

Actually in my company one field require from which date the car allowance is started.

For that one date field is require on PA30 screen ..

Also suggest me which screen is suggestable for that. and also give me any othe way is possible.

Thanks & Regards

Anubhav