AMO After Market Order is possible. You just need to place a counter opposite order.
Post Market Order (Do you mean the trading during 3:40 to 4:00 pm), In this session it is not possible to square off. Trading in F&O closes at 3:30 pm for EQ derivatives.
You can place FNO entry or exit orders using AMO (after market orders) facility in pre market timings and after market timings from 6:30 pm to 9:15 am. But they are submitted to the exchange only when market opens at 9:15 am.
I mean, Pre-market order (between 9 & 9.08 AM)
Derivatives (F&O) are not allowed in PreMarket 9:00 to 8:08 am.
So not possible to square off during that time.