It's theoretically possible to do so but I doubt any software implements this currently.
mRAW and sRAW are both ways to compress images that attempt to preserve as much if the editing latitude as possible. If you've decided a group of images aren't worth preserving in RAW but you have processed them then JPEG is a much better space saving strategy preserving both resolution and image quality (but not editing latitude).
The desire to convert to m/sRAW at a later date just doesn't make sense to me: you've decided that a set of images are not worth preserving at RAW filesizes & printing at high resolution, but you want to preserve the ability to change your mind about white balance at a later date. If you think you might reprocess images to make them better then it seems worth investing in small increase in storage space to keep the original RAWs.