I think you are asking me how to add Pokemon to your party, as in your own Pokemon. There should be a map called Pokemon Lab in Essentials when you open it. In that map, there should be a table with three pokeballs on it. Go to the event and layer, right click one of the pokeballs, and click Edit. Here you can see how starter pokemon events are done. There is a part of the event that should say something like Script: (blahblahblah) [pokemon name here],5
You can double click that event to open up the script. So say you want to change that Pokemon to a Mudkip. Erase whatever pokemons name is currently there and type, in caps, MUDKIP. (make sure you do not erase ANYTHING besides the pokemons name when you edit the script. The number after the comma is that Pokemon's level when you get it.
If this is not what you meant then please clarify a bit more.