Hello,
this is not possible in a standard Alfresco environment. The core SiteService expects all sites to be located in a flat list in the same directory / container. There are also no UI components that would be able to deal with nested sites in a way that an end user might expect, e.g. aggregating content from all parent and child sites in the view of the document library (I have heard such requests from customers in the past).
Regards
Axel
