Answer:
Download the character you want to add as a ".zip" or ".rar" file.
Extract the files from the archive to a folder on your computer.
Open the Super Smash Bros. Crusade folder and locate the "fighters" folder.
In the fighters folder, create a new folder with the name of the character you want to add.
Copy the extracted files from the character archive into the new folder you just created.
Start Super Smash Bros. Crusade and go to the character select screen.
If the character you added is not automatically unlocked, you may need to unlock them by completing certain objectives in the game.
Step-by-step explanation: