You have to modify spawn block actions through xml editing to spawn terrain.

<groundType>40</groundType>

There is Spawn Block Action
It allows you spawn Barrels, steel, crate and cages (if I remember well...)

And as Soup said, you can change the block which is spawning

You can spawn any terrain, my tool can mark a whole area for spawning (check my guide) here is an example:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=401000223
The bigger problems are that blocks often wont connect, backgrounds also wont spawn proberly, also the least bugs are happening if u spawn all blocks at once (so they connect) but this also means the game has to sync it all in one frame, which leads to framelag and might have consequences online (not tested)
The background thingy might be solveable but i need more coding for that i guess

Feel free to check the level and my tool :)

/edit: I uploaded a screenshot on steam a long while ago which shows all blocks next to each other:
http://images.akamai.steamusercontent.com/ugc/527257229458144250/84A99499BCAA1509448C7D3C31CF37768E08B7CF/?interpolation=lanczos-none&output-format=jpeg&output-quality=95&fit=inside|2048:406&composite-to=*,*|2048:406&background-color=black
I will have a look if i can find my coded list of block names (not sure if i backed it up from my old pc)



/edit2: http://pastebin.com/RwKQYjcs Here you go, thats how my tool translates atm (terrain name from xml level format, not ingame[they differ], with corresponding ID) If you find out new once please notify me directly or post here :) The ID 4 is one of many "Empty" blocks at that time (as you can see on t he screenshots there are many gaps), not sure if it will stay this way. I just chose the first one. Also be aware that every different tileset (Jungle City hell etc) does have some slightly different blocks and the list doesnt fully account for all nor does it list the backgrounds that you can spawn (as seen in image)
And yes, you can spawn standard mooks, for whatever reason they have a block ID^^

Interesting, thanks.