iText - How to replace a character with a dynamic checkbox in a pdf

I'm using iText to working on pdf documents. I have a pdf document with non-dynamic checkboxes (they are made with the character "□", unicode \u25A1); I need to replace that checkboxes with the dynamic checkboxes of iText. It's possible? I need it because the pdf document that I receive is not made by me and it contains that character. Thanks for the help, and sorry for my not-perfect english