how do I create a stamp that functions like a fillable form?

Your stamp will not have fillable fields - a stamp is a static piece of content in your PDF file. The way dynamic stamps work is by popping up a dialog at the time you place your stamp that asks for the information that will be placed on the stamp.

At the end, a digital stamp is the same thing as an old fashioned ink stamp: Once it's on the page, you can no longer modify it.

Take a look at this tutorial, it explains how this is done: http://acrobatusers.com/tutorials/dynamic_stamp_secrets

Karl Heinz Kremer
PDF Acrobatics Without a Net
PDF Software Development, Training and More...
http://www.khkonsulting.com

What about

event.value = event.source.source.getField("Fld Name in Received Pdf").value;

to set the value of a Dynamic Stamp's field with one from the received doc?
Additionally, you could think about using app.setInterval to make the above setting effective only when the field of the received document has been filled in.

I can't assure it is going to work. You need to test it.

According to this thread:
http://answers.acrobatusers.com/Dynamic-stamps-q142813.aspx?tc=792360232082002201493607
it seems that app.setInterval is not applicable to dynamic stamps (both within their Calculation Scripts and from the host document being stamped).

From this article http://acrobatusers.com/tutorials/dynamic_stamp_secrets:

To make the stamp dynamic you add form fields to it. At least one of the added form fields must have a calculation script. Acrobat runs the stamp calculation script every time it accesses the Stamp File. This means the script is run when Acrobat first opens the Stamp File, when the stamp is displayed on the Stamp Menu, and when any stamp in the Stamp File is placed on a PDF.

In other words: Acrobat ALWAYS runs the stamp calculation script BEFORE (can happen several times on different events) the stamp is placed. Once applied it is just like a static image.