You can't create custom stamps with Adobe Reader.
I wanted to propose an alternative answer. While you cannot take the necessary steps in Reader to create a dynamic custom stamp, it can be done. If you know someone with Adobe Pro, they can create the stamp for you and send you the file. Here's how you do it.
1. Create a Custom stamp in Adobe Pro (use this forum http://www.acrobatusers.com/tutorials/2007/02/custom_dynamic_stamp, it's the best description by far).
2. After you've added in the dymic properties and reach the end of that forum; take the file mkWXAu6ok-vDmYeXdpd4EB.pdf (or something crazy like that) from location c:\Documents and Settings\login name\Application Data\Adobe\Acrobat\9.0\Stamps, and send it in an email or put it in a network folder. On side note: To find that folder you need to go to My computer, go to Tools>Folder Options, select View and click the radio button for "Show hidden files and folders. This is neccessary because Application Data is a hidden folder. Also when you are first saving the file mkWXAu6ok-vDmYeXdpd4EB.pdf, it will be read only, so right click on the file and uncheck read-only so you can save the file.
3. On the Adobe Reader computer you need to go to the same location c:\Documents and Settings\login name\Application Data\Adobe\Acrobat\9.0\Stamps and just copy the file out of your email or network folder. You will now see these files in your stamps on Reader 9.0.