You can access PostgreSQL tables in Microsoft Access via ODBC. It's a built-in function of Access to connect to other ODBC databases. Use the PostgreSQL ODBC driver. Then copy the data in Microsoft Access to the PostgreSQL tables. You can also use the Microsoft SQL Server Import And Export Data Wizard to copy data from one database to another. All you need is a valid OleDB or ODBC connection to each database.
But, when you need to copy data from Access to a Mendix-PostgreSQL database, then it's better to create a custom Java action, make a connection to Access with the JdbcOdbc-bridge driver from Sun, read the data and use the normal Mendix methods to store the data in the PostgreSQL database.
I've found an explanation how to convert a MsAccess database into a PostgreSQL database here.
The example above contains the following five stages:
Stage 1 - On your Workstation running MS Access
Stage 2 - On your Linux Server running PostgreSQL
Stage 3 - Testing your database at the server console.
Stage 4 - ODBC Configuration
Stage 5 - MS Access Configuration.
Another solution can be found here.
If you're using Windows instead of Linux, perhaps this link will help you out.
An ODBC postgresql driver can be found here.