I may be wrong, but I believe what you're looking for is already available in the References tab for the Tables node:
There you can select which tables to include and both imported and exported keys are shown.
Great! Didn't know about this feature (in the tables node)... It would be nice though to make it a bit easier, and more interactive. It seems that the only way to add tables, related by external key, is basically to know about the relation.
It would be really nice to make the relation graph more exploratory, for example by right-clicking a table in the graph and add its related tables (by imported and/or exported) keys..
I have added your comments about improvements. We have lots of ideas for this area, but it requires quite a bit of work and has not yet been scheduled for a release. You will be notified if we release improvements in a future version.
Also, are you aware of the Navigator feature?
It would be really nice if additional referenced tables could be added to the reference graph.
In the reference graph for table A, all tables with references to table A is listed (B & C).
I would like to add tables with references to table B, (or C) as well, to the graph.
This, combined with an option to show imported together with exported keys, would be a really nice way to explore the db.