How does imposing 100 percent cash margin requirement on imports help to stabilize foreign exchange reserves of a country?
Simply because when you want to buy goods from country A, you (generally) first have to buy A's currency, which means selling yours.

And given that selling a currency exerts a downward pressure on its value, the main idea behind a 100 percent cash margin requirement is about counter-balancing the depreciation by exerting an inversely-proportional upward pressure.

That being explained, if importators do so by mobilizing already-owned and in-excess cash, there is no reason for such a policy to work... which is unlikely, say, in the case of Turkey, since domestic liquidity have already been sold, which means buying them back.