The current AddToOutlook.aspx page does not support putting multiple vEvent items in one response. However, you can implement such functionality quite easily.
To start with, have a look at the code behind of the page (AddToOutlook.aspx.cs). Then create your own page (modifying the current one is not recommended due to possible issues during upgrade). The page will accept a query parameter specifying the node alias path from which you want to load the events. Using this path, select appropriate nodes from the content tree (you can find inspiration on how to select document nodes in http://devnet.kentico.com/articles/kentico-8-technology-documentquery-api ). Keep in mind the security aspect - do not process alias paths which would disclose anything else then public events.
Once you have the nodes, implement a method similar to GetContent method from AddToOutlook.aspx.cs. All you have to do is add multiple vEvent items in the content of the response (each is delimited by BEGIN:vEvent and END:vEvent).
Shall you need more information regarding the vEvent format, consult the following RFCs:
Regarding the webcal protocol handling, I am not really familiar with it, but I think you will have to configure a proper bining in your IIS (to respond to other than http(s) protocols).