at the moment there is no (easy) way. There is going to be a proper API for that in API3.0.0 I think you can expect it somewhat soon.
You may take a look at the source code of this plugin: https://github.com/BoxOfDevs/CommandUsage That could probably help
If you registered the command, you don't need to override packet sending like CommandUsage. Instead, you can override generateCustomCommandData in the Command class implementation. This theory is not proved though.
Those are some OP hacks. Registering "login <password>" as a command isn't the solution. I can't believe someone capable of making such a plugin happens to be a bad code formatter and writer :3
@Ad5001 makes good plugins, that one just happens to not be formatted as well. I'm also pretty sure that it doesnt work with the latest api3/network branch
#BlameErkam https://github.com/BoxOfDevs/CommandUsage/commit/35c2915aecc9b503e5027f3f8760ec0e9e3d3a5a I did format it right D: