To do this you will need superconverter or any video editor able to extract audio track from a video and to save video to jpeg, THPConv.exe, dsptool.dll and thpplay.
If you want to use superconverter, take care to decline the installation of the meteo application and the pdf reader proposed (useless as always).
Superconverter: http://www.erightsoft.org/GetFile.php?SUPERsetup.exe Thp tools containing THPCon and thpplay: http://www.mediafire.com/?ame8j1j4rn5hvhe
The process below is based on this excellent tuto http://www.smashboards.com/showthread.php?t=271087 and the THP Library guide I advise you strongly to read.
1) Extract audio track from your video. Use any soft you like which is able to do that (vlc, virtualdub, Avidemux...). I use Avidemux but you can use superconverter as well. You can adjust the volume audio with a sound editor if you think it's too low.
The most important is to save the audio in WAV 16 bit PCM Mono or Stereo or THPconv will output an error.
2) Convert your video to a series of jpeg files. Before to start it is important to know what is supported by THPconv. Sequential JPEG files. The width/height of the image must be a multiple of 16. The maximum number of pixels in the horizontal direction is 672, there is no restriction in vertical direction.
After taking care of these recommendations, use superconverter or your favourite video editor.
With superconverter, right click, add multimedia files, select your video. Double click on the file name, "The multimedia Analysis box" appear, it shows informations related to your video. The information that interested us is the frame rate. Note this value, you will use it later with THPConv and the -r option.
In select the output container, choose sequence of images. In select the ouput Video Codec,choose JPG. In video scale size select 640*480. It seems that a lot of Wii games use this resolution for PAL but I really don't know. Frame/sec use preferably the same as your video. Click encode
3) Using sequential JPEG files and wav file to create your THP movie data. At this point you should have the wav file, and a huge amount of sequential jpeg files. Put in the same directory: THPconv.exe and dsptool.dll, the wav file and the series of images (make sure you have .jpg or .jpeg extension).
In command line use:
THPconv.exe -j *.jpg -d video.thp -s sound.wav -r 24 (this last value should be based on your video source)
-j specifies the input jpeg files (keep *.jpg) -d specifies the output thp file -s specifies the audio file (wav) you extracted from your video -r specifies the movie framerate, the default value is 29,97. To avoid an out-of-sync audio/video, use the same frame rate as your video source. This value can be found in the multimedia analysis box or in any video editor.
4) Check the quality of your thp movie with thplay. Open thpplay and drag and drop your new thp file to check the final result. I converted a short video and the quality was not bad.
I am not a video specialist and I never used this method to replace video on Wii games, so experiment and look what is the best.