From my little research, it doesn't appear that Excel 2007 can parse and interpret JSON. A developer could build a service to translate the data from MtGox, etc to a format that Excel 2007 can use. How much of a PITA is this to you? How often do you need to retrieve data from MtGox and others?

For reference purposes, you can read about the Excel feature here: http://office.microsoft.com/en-001/excel-help/get-external-data-from-a-web-page-HA010218472.aspx