I am not aware of any for 3ds to DS, indeed there not that many 3d hacks in general for the DS (even fewer if you exclude simple rename/overwrite type things). Most of them will be things like remaking old levels and tracks in things like Tony Hawk (the "ultimate" version available on the PC as a collection of mods for example) or Mario Kart Wii (a lot of the Crash Bandicoot kart racers got things ported over). Occasionally you might get one for a fighting game, though often that is more cosmetic and more in line with the model tweaking stuff.
Generally such things revolve around having an understanding of at least the files used in the games in question (there might be aspects of the format seen on other games that are not needed/used as thus be unnecessary to know about) on both systems, possibly some about the methods used (so here you would want to know the rudiments of 3d graphics and at least the basics of how the hardware works as far as what sorts of numbers and types of animations favoured) and translating things across.
If someone made nice import and export filters for things then it gets far easier as you can simplify and redefine numbers to get it there, however as not much in the way of 3d creation stuff for the DS exists you are going to have to go manual.