No, it would overcharge and ruin the battery.
#2 You can do it by using a series resistor to limit the current.
The resistance needed is equal to the voltage drop divided by the charging current.
The voltage drop is 4, and the charging current is one tenth the battery capacity, for example for a 2000 mAh battery the charging current is 200 mA.
So the resistance needed is 4/0.2 or 20 ohms. The power dissipated in the resistor is 4 x 0.2, or 0.8 watts, so you need a 20 ohm 1-watt resistor.
The battery should be charged for at least 10 hours if fully discharged, or until it becomes slightly warm to the touch. Do NOT let it overheat.
Charge it with a 12 volt battery charger.
No, a 12 volt charger will overcharge a 8 volt battery and destroy it.
A 12 volt battery charger is designed to charge 12 volt batteries.The answer to the Related Question explains how any battery charger works.
No, the battery charger has to match the voltage of the battery it is to charge.
If you are asking the question, can you charge a 6 volt battery with a 12 volt charger, the answer is no.
No you cannot.
no it can't
No it will not work
No, an automobile battery charger outputs 12 Volts. This is far too many volts to charge a 5 Volt battery. You would destroy the 5 Volt battery with this charger.