Do I need to have a free margin to lock positions in my trading account?

No, when you are trying to completely lock (hedge) a position, which typically involves opening an opposite trade to an existing one (e.g., buying and selling the same instrument) with the same lots, it would offset the margin of the existing one. However, if you open an opposite position with different lots, then you are still required for partial margin amount.

This can be referred for each product in product specification, "Margin hedge" – size of margin for hedged positions, which typically is 0.