There's a couple of programs that'll do it if you google around. Unfortunately, it's limited to 20fps and 10min.How?
Awesome! I actually made a post asking about movies a while ago but got the general answer that it wasn't possible. So happy now, thanks!https://gbatemp.net/threads/3ds-video-beta.315365/
3DSExplorer can do it to, IIRC, but I used this for some Whose Line is it Anyways? videos way back in the day, and it worked well.
You'll need to place them in the DCIM folder along with your pictures, but once converted through that, you'll be able to play 'em.