As an alternative to migrating such Postgress database, you may want to go for a co-existence approach (which eventually you can still migrate later on). To do so, have a look at the Forena module (disclosure: I'm a co-maintainer of it). It can be used to build reports and/or charts about data stored in your existing Postgress database. Here is a quote about Forena:
... built of the idea of using SQL to get data out of a database and use XHTML and CSS to format it into web reports.
The advantage of this approach is that you get lots of Drupal features available, while the "backend-part" of your application continues to be the applicationn you already have (you don't loose the investements made in that).