I've had some issues with this. First there are certain order types that by default won't execute outside normal trading hours. There is a window where you can change this.
Second, sometimes when I place overnight orders for t-note options on ecbot I get a popup message that the order won't be placed on the exchange until morning. But then my order (usually) shows up on my screen and sometimes gets executed, so I am guessing the message is a mistake ( I doubt they are crossing my order with another customer but I never cared enough to confirm this.
I don't remember having to get my account enabled for overnight trading but I can't swear I didn't check a box on a form when I opened the account or something.