Google Finance supports two ways of getting at your data, CSV and OFX:
CSV files are a simple format for storing spreadsheet like information. To export a portfolio as a CSV file, view the portfolio at your Portfolio page and click the Download to Spreadsheet link. That's it! You should now have a file called data.csv in your downloads directory. You can view it in any spread sheet program, such as Google Spreadsheets.
OFX is a format that works well for exchanging financial data. To export your portfolio has an OFX file, just view the portfolio at your Portfolio page and click the Download to OFX link. You should now have a file called data.ofx in your downloads directory.
Many financial institutions will let your download your portfolio as an OFX file that you can import to Google Finance.
To do this, go to your Portfolio page and click the Create a new Portfolio link at the upper right. You will be prompted to provide a name for the new portfolio. Now you can now either add securities and transactions manually, or click the Import Transactions link, again in the upper right. Select an OFX (or CSV) file and click "preview import". If everything looks good, click the Import button and you're done.