You will need a way to edit SQLite database files, such as SQLite Database Browser.
The database file you want to edit is located at
UserData/Database/boxee_catalog.db.
Linux: ~/.boxee/UserData/
Mac OS/X: ~/Library/Application Support/BOXEE/UserData/
AppleTV: ~/Library/Application\ Support/BOXEE/UserData/
Windows Vista: C:\Users\
Windows XP: C:\Documents and Settings\
IMPORTANT: Back up the boxee_catalog file before editing!!
You will want to edit the video_files table.
Add a new row to the table. The idVideo value should automatically increment to the next available value, but you may want to verify this.
You do not have to fill in all the fields. These are the fields I used. I didn't really experiment to find out which ones were required.
Start up Boxee, and the new movie appears!