Add:
has_cart = true;
cart_sprite = sprite_get("mrsys_cartridge");
into your character's init.gml

and Add:
sprite_change_offset("mrsys_cartridge", 8, 8);
into your character's load.gml

Make sure that you use the template images included within his files (remove the "template0X_" at the beginning).
NOTE: Remember that the Cartridge shell of a Famicom Cart can be different colors, while NES Carts are usually either Gray, or Gold.
NOTE 2: While you can make him hold anything, keeping it to the Cartridges would be more on theme with him.

Built-in compat includes:
SpongeBob
Mega Man
Red
Mari REDUX
Freddy
Sonic
Crash
Dingodile
Super Mario
64 Mario
Gangster Mario
Bart Sickson
Homer
Doc Robot
Proto Man
Cuphead