Good question!
There are two ways to import an existing site or web application:
1. Use Bulk Import. In the web client, navigate into a sandbox. Under the Actions menu, select Bulk Import. You can then upload an existing zip or tar file (if a static HTML site or PHP or .NET or other type of site) or WAR file (if a JSP-driven site). Alfresco will automatically unpack this uploaded file and return the list of newly created files and directories to your Modified Items list where some or all of the assets can be submitted to staging and snapshoted.
Note that if a WAR file specifically is expanded, the virtualization server will be notified and automatically load a new preview context for your sandbox (and similarly do so when those assets are committed to staging).
2. Use CIFS. You can mount your web projects - including all sandboxes and staging snapshots - via CIFS (\\servermachine_a\avm). There, you can simple drag-and-drop any existing site or web app directly into your sandbox, where once again it will show up on your modified items list and be available for preview and/or submission.
The CIFS interface in particular makes using Alfresco good for an existing site. Really, we can manage any type of site, regardless of whether it's PHP-driven, .NET, etc. Also, you don't have to change the tools your developers or business users are using (though likely you should over time cut them over to use web forms to create structured XML for purposes of content reuse and multi-channel publishing) and - most importantly - you don't need to recode any asset references.
Let us know how you get on.
Kevin