How to include input boxes in Tcode FS00

Hi Experts,

I need to include two more input boxes in Tcode FS00 .

<b>Already Exits one is in Tcode FS00</b>

GL Account number (one input box,length 10)

<b>Our current requirement is in Tcode FS00</b>

GL Account number (one input box1,length 10) (input box2 ,length 3) (input box3 ,length 3)

So how to design the Screen (Tcode FS00) to include input box2 and input box3 in that screen. Can please anyone give me steps detail.

Thanks.