No. you cant join in the middle of a duel.
IF the battle is a clan event. (Like a fight for a system, etc) you cant join in the middle of it.
If the fight is something like a player being attacked by other player (and it is not a duel or a faction event) you can join in the middle of the battle to defend/help in the attack, if at least 1 of the players (or sides, if there are more than 2 players) who are fighting agrees to that.
(If you find any mistake in my English, please let me know via a PM)
(Really, I speak terrible English, so please, tell me if I make mistakes. I'd like to improve it a bit )