[anybox]

If you want to do with command you should type this code

pawn Код:

if (!strcmp(cmdtext, "/nitro", true)) {
    if (!IsPlayerInAnyVehicle(playerid)) return 1;
    new vehicleid = GetPlayerVehicleID(playerid);
    AddVehicleComponent(vehicleid, 1010); // X10 nitro
    return 1;
}

But if you want to add nitro to the all vehicles you should write this in the end of OnGameModeInit

pawn Код:

for (new i = 0; i < MAX_VEHICLES; i++)
    AddVehicleComponent(i, 1010);

If you have a variable contains last vehicle id (for example, lastVehicleId) you can rewrite your loop as
for (new i = 0; i < lastVehicleId; i++)...