Question:

Can I join the event table and origin table using "prefor" as a key in dbe?

Answer:

There's not a way in dbe to specify alternate join keys; you're pretty much stuck with the natural join keys.

However, you can create a "canned" view in the .dbe.pf file which does override the default join keys, as I've done below, adding the Event view:

  views               &Arr{
      Site    &Tbl{
      dbopen sensor
      dbjoin sitechan
      dbjoin site
      dbjoin instrument
      }
      Origins &Tbl{
      dbopen assoc
      dbjoin arrival
      dbjoin origin
      dbjoin site
      }
      Arrivals &Tbl{
      dbopen arrival
      dbsort time
      }
      Event &Tbl{
      dbopen event
      dbjoin origin prefor\#orid
      dbjoin assoc 
      dbjoin arrival
      dbjoin site
      }
 }

Then you can open the Event view from the File->Open View menu.

You can also just join event and origin, and then subset "orid == prefor".