The Extension Mappings came along in release 8.1.1, about a year ago, and are in the regular documentation.....we can discuss them further here, and I also have a few entries on them in my blog (link below).
The documentation is also easily accessible on the web at
http://publib.boulder.ibm.com/infocente ... /index.jsp ...just search for "Extension Mappings" and all the sub-topics will appear.
You can most certainly create Extension Mappings via FastTrack by exporting the FastTrack Mapping and then importing it directly into Workbench.
Shared Tables, once created, can be enhanced in a lot of ways....you can edit a short and long description, qualify the table with an image, define a subject matter expert (Data Steward), add extra "Notes", assign business terms, etc...
Also, I just added a blog entry on "What are Shared Tables" that you may find interesting.
Ernie