First off all I want to say hello 
I’m new to JME and certainly gonne play around with it to eventually create a game I have in mind for a long long time.
I’m use to mapping for games like half-life and half-life 2, and wondering if there is any tools to create a map that looks simular like hammer editor valve has.
I’m trying to create a fps game with maps like the ones hl uses.
Now i’m trying to find out what would be the best way to create maps ‘simular’ like that.
Cause I don’t need maps like terrain but just indoor maps and offcourse later in the development I would need outside to.
Hope somone can give me a direction I could look into
Simple FPS Creator seems like the exact thing you are looking for. http://lmgtfy.com/?q=Simple+FPS+Creator. Nah just joking 
The jMonkeyPlatform has a Terrain Editor. Take a peek in the docs. That could sure proove useful for outdoor terrains; unfortunately i wouldnt know about an easy way to create indoor levels.
My first guess is to build your level using the SceneComposer in jMonkeyPlatform: In the simplest case, creating Planes for walls and floors and sticking them together and adding materials to them. Of course that means manual work, and thus not a tool-approach exactly.