FOREIGN KEY after CREATE TABLE?

How can i add foreign keys after the CREATE TABLE? Is there some
combination of other SQL commands that will do the trick?

My problem is;

a. that I have a circular reference between 3 tables
b. that I must be able to reverse engineer the resulting references in
the shema by querying for foreign keys

Right now I am calculating a dependency hierarchy and generating tables
in the resulting order but this does not work for a circular ref.

Aside from the circular ref issues... i think it would be easier and
safer to be able to generate all of the tables in one pass (with primary
key defs) and then add the foreign keys in a second pass.

Any help appreciated.

Peter