If it's a statutory fire door you cannot lock it. However, I suspect this is just a door between garage and house which happens to have a fire retardant rating (as if should), because garages can be a fire risk considering the fuel, oil, grease etc which can be in there. Yes, you can fit a lock or bolt, it will just require a little more effort to install, i.e. using bolts or self-taooers instead of the usual wood screws. Make sure it can be easily opened from the house side. Try asking a local handyman (ask your neighbors for recommendations).