There are a lot of missing pieces to this problem. Outlook doesn't directly interface with SharePoint aside from synchronizing list items to an offline list.
I think you may be looking at this the wrong way. Your XML document isn't an application - it can "connect" to the calendar however you want to, depending on the application consuming it. It sounds to me like you want the connection to go the other way - you want the calendar to connect to your document and update based on the contents of the XML file.
An event receiver on ItemAdded/ItemUpdated can parse your XML file, check for matching events, and add/update those events to a SharePoint calendar appropriately, and then one or more users of Outlook can import that calendar.
I don't see any way this can be done without code, I'm afraid.