to be able to create database link from Oracle's database to non Oracle database you have to use additional product which called "Oracle Database Gateways".
Or as another way it's via some BI tool which performs join of result set of different database on its side.
Hope it help.
P.S. In scope of SQL Developer, It just issues SQL to database but the query executed in DB.
So in terms of SQL Developer, I believe it is impossible, at least out of the box.
Thx . I actually got it to work, I had to just create dblinks to each postgre db and setup the db's so that sql dev could access them via odbc connections.