Stop loss after a normal order has been executed

Can we place a stop loss order after a trade has been executed in normal order, if yes how?

You can place NRML SL or SL-M order, but if this order is not executed it will be automatically cancelled by EOD, you cannot carry it forward to the next day, you can read this to know how to place Stoploss order.

For overnight positions you can use GTT orders to place Stoploss, you can learn more about GTT here.