Hi!
Unfortunately there is not a way to add a custom stamp to the Stamp drop-down list in the current release of DWF Composer, but your feedback helps us to consider feature enhancements for the future.
You can, however, edit any of the standard Stamps to create your own Custom stamp(s) that you can then Copy/Paste into other sheets or DWF files.
To create your own custom stamp:
1) Invoke the Stamp tool, select any of the standard stamps and add the stamp to your drawing sheet.
2) Invoke the Selector tool and double-click on the stamp object to enter into text-edit mode.
3) Edit the text to read whatever you like, then press ESC to exit from text-edit mode.
4) Select the stamp again (single-click) and apply text, fill, and/or line formatting for the stamp as you like.
Hope this helps. Thanks!
Scott Lillibridge
Autodesk, Inc -- Viewers