The models in the MariaEngine64 are all generated from code (path + config parameters + texture atlas), this is done in the skelani_init_all_models script. After you've defined a new model go to obj_player's create event and replace the line "skelani_load_model(smd_MARIA)" with loading the other model you want.
For external models, though... there's an OBJ (Wavefront) importer script in the open world project you could just copypaste over but there's sadly no support for using existing skeletal animation information in the model (Game Maker doesn't support skeletal animation natively so I had to make my own separate system)
If you're using existing model assets you might be interested in Snidr's Model Format, it's a free skeletal animation system for Game Maker. It's a bit tricky to set up (and requires you to convert the models to a different, custom format) but there's very few other options.