My suggestion is for a tutorial to be made for creating modelcustomization files.
I appreciate the suggestion and desire to modify iiMii Maker to import your own assets. I haven't originally created this tutorial because I wanted to be sure people were actually interested in doing so. (It's hard enough getting people interested in using the app as it is) The method of doing so is actually a bit non-user friendly as the existence of the modelcustomization file is mostly just intended to dodge any DRM notices by Nintendo.
This tutorial may not happen anytime soon because to modify your own assets require you to be able to create an AssetBundle which is a Unity Editor-specific feature at the moment. (Sort of like many Unreal Engine mods requiring to create the mod using Unreal Engine itself for building it into the proper mod format.) Which isn't necessarily an issue in itself since many game mods require some adverse applications just to develop them but I'd rather go in and make creating your own modelcustomization file more convenient which will require another update to iiMii Maker and then make a tutorial for THAT instead of making a tutorial for the current, quite difficult, method. That takes a lot of time and effort to develop, however, and as of now you're quite literally the ONLY person that has even mentioned modifying the modelcustomization. (Even though I tried letting people know that's what it's for - to customize in-game assets)
In the future I may create a more robust, user-friendly method of making your own modelcustomization file but, as of now, creating a tutorial for something of such low demand takes time I sadly don't possess. :(