Hi,
The only way to dynamically refresh a spreadsheet is using the Refresh button that the dynamic worksheets give you - there is not out of the box workflow activity that can do this for you.
If you are using Dynamics CRM OnPrem you might consider using Reporting Services to create a report (using the Report Wizard) and then schedule the report to create a snapshot. These snapshots can then be exported to Excel by the user if required. You could even configure it to email the report off if required.
See - www.powerobjects.com/.../report-scheduling-and-email-subscription-in-crm-2011
If you are using Online then this technique will not work and you could consider using a third party report scheduler solution - community.dynamics.com/.../report-scheduling-and-email-subscription-for-microsoft-dynamics-crm-2011-online-and-on-premise.aspx
hth,
Scott