Notice: This article may contain information for multiple versions of Passpack, be sure and read the section appropriate to your version. How do I know what version I'm using?
Passpack v7
Firefox
You could use this script or, much easier, install the Password Exporter, a dedicated Firefox extension. It allows you to export your saved passwords to an XML file. You'll need to convert that to a CSV file, then follow the instructions to import into Passpack.
Explorer
IE Password View to extract all the passwords stored in your browser, and can export into several formats, including tab-delimited text files (TSV) and XML files. Then follow the instructions to import into Passpack..
Passpack v8 (Preview)