使用PDFTK,输入pdf不是一个acroform,如何使它成为一个acroform

I am using pdftk in my web app.. first I am merging 3 pdf templates with fillable forms into one pdf template with fillable forms (let's call it 'simple.pdf').

Then, I want to go further and fill in 'simple.pdf's custom fields but I am getting the warning 'input PDF is not an acroform, so its fields were not filled'.. is it possible to preserve the 'acroform' attribute ?

Thanks !