Appending to PDF form fields using iText APIs

I have a PDf to which I need to append data at specific locations. The way the PDF is is that it has form fields. I need to fill those fields appending to an existing PDF and save the newly created PDF.
I am trying to using the iText API for Java to do that but I cant figure a way to use them for this purpose.
Has any one done that .Please post a small code snippet as to how I can go about doing this.
Please help me out.

Thanks
Dhiren

Basically, you can't. iText is an API for creating PDF files, not for working with existing ones. There are very few operations iText (or other Java PDF libraries) can do with an existing PDF, and filling out forms is not one of them.

is there a Java API which works with existing PDF ?

Thanks
Dhiren