Creating mjpeg,adpcm AVIs?

Hi there --- I guess this is a somewhat academic question, but...

Can anyone tell me if and how transcode (perhaps via ffmpeg, avimerge 
etc.) can *encode* AVI files as mjpeg video with ima_adpcm audio? Why do 
I want to do this? Well, I'm curious to see whether I can create files 
in the same format as those produced by a Casio Exilim digital camera 
such that they can be copied to the SD card and the camera will play them.

So far I can create files whose video content does play on the camera...

transcode -i somexvidfile.avi -o camfile.avi -Z 320x240 -y ffmpeg -F mjpeg

but whose audio is still the mp3 encoded 2 channel 44100Hz of the 
original. The target audio is (I think) 1 channel 22050Hz encoded as 
ima-adpcm. I've not been able to figure out how I can get ffmpeg to 
create such audio, or how I can get transcode to use ffmpeg to do it 
even if it could. However, the audio can be extracted like this...

trasncode -i somexvidfile.avi -o camfile_pcm.wav -E 22050,0,1 -x 
null,raw -y wav

and sox used to convert from pcm to ima_adpcm...

sox camfile_pcm.wav -i camfile_imaadpcm.wav

and an avi with video but no sound is easy enough to create.

But... that's as far as I've got --- can transcode create an AVI that 
combines my camfile.avi and camfile_imaadpcm.wav?

cheers
guy