Hi Kit,
There is no functionality to export a database model diagram from within Workflow Manager. We do provide a database diagram in the installation, however, it is WMX System Tables.pdf under the Help folder of the Workflow Manager install directory. You didn't specify which version of our product you were using, but for 10.3 onwards we have updated its styling to make it easier to interpret and I can provide you a copy if you would like.
External custom tables (Ie. Extended Property tables) are joined to the Workflow Manager database using the JobID field, the central table for job information is the JTX_JOBS table.
Cheers,
Michael