If your knockout has a Plate event, typically for first round losers to play, the best way to do this is to enter the page for the Main event and click on the Make Plate Event button at the top. Once you have filled in the new name, you will have a new knockout in your list of events.
This behaves like any other event, but now when you go into the Main event you will see a new button at the top to Copy Teams to Plate Event. Pressing this opens up a page like the following:
Here you can select exactly who you want to copy over from the Main event to the Plate event. Typically you only want to copy the losing teams from Round 1 or sometimes also from Round 2, but you have the option to copy winners and losers from any round, just in case you're doing something strange. There are two further options:
Teams who have won a match in another round do not count as losers means that if you have byes in round 1 and you choose to copy over the losers from Round 2 then only the teams which had a bye and then lost would be copied over, not the teams which won and then lost.
Walkovers do not count as match wins for the purposes of the above rule means that if a team won a match by Walkover and then lost in round 2, they would be copied across.
Once a team has been copied across it gets marked in the database as such, so you can repeat this process when you have more results without worrying about duplicating the teams. This also means that if you delete a team from the Plate event it will not be reinstated with the next copy action.