XBMC includes a fully-functional file browser. Here's a brief set of screen captures showing how you can copy a file from a DVD or CD to a new folder on your Xbox hard drive.
Step 1: Open "My Files"

From the main page of XBMC, open "My Files" as depicted in the diagram.
Step 2: Create the folder

Navigate to where you want to create a folder on your Xbox HD in the right pane. I've gone to the root of the E drive in this example. Press the
white button to bring up a menu, then press "Make New Folder" and proceed to give it an appropriate name.
Step 3: Copy the file
Navigate to the file(s) in the left pane that you want to copy to the newly created folder (E:/video in my example) and press the white button again to bring up a menu. Select "Copy" and the file will be copied to the Xbox hard drive in the right pane. Repeat as necessary.