Your question is confusing. Are you looking for an ETL tool? Most connect to heterogeneous sources (i.e. Oracle, SQL Server, flat files, etc.) These tools also allow you to examine the structure of the source system. Informatica and SSIS are two that come to mind. Profiling data may take some manual effort via queries in the source system.
When you say plain txt for DDL: Any RDBMS will allow you to export the DDL of tables. .sql files should be usable, but be aware of datatype differences between RDBMS.
If you are provided dumps or backups you must restore the backups or dumps into the appropriate RDBMS. From the restored databases you will be able to design an ETL into Postgres. Your ETL could be as simple as dump all the source tables to CSV files. Oracle and SQL Server both have excellent command line export utilities.