You are wanting to merge all of your forms? Or is this a choice within a form? I am not aware of a select all in SharePoint for this. Frankly I have not seen merge really used effectively for a number of reasons.
Typically if you want to report on or aggregate all of the data from your forms you need to collect it in a backend database. That way all of the data is accessible realtime via Excel or SharePoint.
Email me if you would like to see an example.