Using car charger for small sealed-type lead-acid batteries
Is it possible that I use the said charger for the said batteries?


No, the car chargers current will likely damage your battery.


  If so, what are the steps to modify the charger to suit the batteries?


In theory, a resistor or incandescent light bulb in series to the battery could work. I however suggest charging the battery by connecting it to a constant voltage (13.8 volts for a 12 V battery). A bench power supply then can limit the charging current to 0.4A.

This method is obviously simple but comes with a price: It is slower than other methods and can decrease the batteries capacity over time. But I really like it because you don't have to worry about overcharging the battery.