NationBuilder lets you upload CSV files to your database (here are some tips on ensuring clean CSV imports). From the control panel, go to People > More > Import.
Most email service providers let you export your contacts to a CSV file. Once you've done that, simply go to the import tab, choose the file from your computer that you wish to upload, and click upload CSV file. Then you'll be able edit the fields that all of your contact's data will be entered to. You can designate the people in this import as prospects, add tags to all of them, and/or add them to a saved list. Click on Start Import to begin the process.
If you are importing multiple files with the same field structure, you can reuse your field mappings.