... inside packages/models/ ?!?!
... via your map.cfg and a call to mmodel, maybe even mapmodelreset
... like look at data/default_map_models.cfg - then load it like any other model, with correct index of course ...
do not expect it to show up in the menu though - that's not dynamic, it's purely hardcoded in data/menus.cfg.
Seriously - you should take a look at any and all cfg's you can put your hands on if you want to get the most out of sauerbraten mapping ... and of course always keep reading and re-reading the documentation, wiki and forum :)