I am afraid there isn't much that can be done. One is to add Signature field using Acrobat. or you may try to Save the pdf document as Reader Extended and see if that may helps : http://help.adobe.com/en_US/acrobat/X/pro/using/WS58a04a822e3e50102bd615109794195ff-7e0d.w .html
I have one suggestion for automating the addition of digital signature fields to your PDF files.
First, for one hint, in Acrobat XI Pro you can convert an Excel file to PDF by dragging it into the Acrobat window.
Below is a sample that places a signature field named "sigfield" at a position 1 inch from the top and 1 inch from the left of an 8-1/2 x 11 document.
The placement is indicated by a rectangle whose corners are specified in rotated user units.
That means 0,0 is at the bottom left of the document and units are points (by default), 72 points per inch.
The corners are indicated in the following order:
[upper-left x, upper-left y, lower-right x, lower-right y]
var name = "sigField";
var type = "signature";
var page = 0;
var mySigRect = [72,720,216,684];
var sf = this.addField(name, type, page,mySigRect);
Thanks, this was the sort of info I was looking for. This will help me automate the process.