Lucy I think he meant to ask if he can ADD anything he wants.
Anyway Realmcrafter is some kind of FPS Creator just for MMORPGs.
You got your different editors to create and manipulate your world/entities/characters. You can create your own assets/characters and create quests through scripts. I don't know how much you can change though, I actually never used Realmcraft. Just saw a Youtube video of it once.
I think Realmcrafter does come with the source, but I'm not sure of it. Just know that some group added some features to it once, bloom etc.
The whole thing is based on Blitz3d which is a DirectX7 engine.