> I am trying to connect via SQLObject to the existing database

  You didn't mention fromDatabase but I am sure that what you meant,
right? Because it's certainly possible to connect to an existing database
without it.

> What I really can not figure out is how to get ForeignKey and MultipleJoin
> working.

  By declaring them yourself:

class MyTable(SQLObject):
   class sqlmeta:
       fromDatabase = True
   fk = ForeignKey('AnotherTable')
   mj = MultipleJoin('AnotherTable', joinColumn='at_id')
 
Thanks for the syntax, it works great.

  Explicit declarations are compatible with fromDatabase. And of course
you are not obliged to use fromDatabase at all - just declare all you
columns and joins yourself.

I have used fromDatabase  because my lazybones only.

  Of course IWBN if your patch fromDatabase machinery (methods
columnsFromSchema and guessClass in Connection class) to recognize at least
ForeignKeys.

IWBN, I would like to. But, to be honest, I think my Pythonic skills are to low for this task (it is REALLY not about the time or laziness).

Thanks and Best Regards

Petr