I have been faced with a similar problem and by using passing &rs:Format=EXCEL it will automatically render a report to excel, i.e.
If you wanted to have the fuinctionality embedded into a report you would just set the relevant field to have the above as a navigation url link. If you wanted to pass a parameter you can use the example below:
="http://localhost/ReportServer/Pages/ReportViewer.aspx?%2fTest%2fReport3&PARAM=" & Fields!ID.Value & "&rs:Command=Render&rs:Format=EXCEL"
This is placed in the navigation properties of the specific ID field that I want passed to the second report.
I hope this helps.