No, looking at ProtocolGame in the sources, the parsing of the PVP mode has been commented out.
However, there are alternatives requiring the use of onAreaCombat() and onTargetCombat() player events.
So, you could have them use a command like !pvp to turn on/off the PVP mode.
Then use onAreaCombat() and onTargetCombat() to check the player's mode and return true/false depending on mode.