Just build it as a wmb. It's the same as building a level.

Then in a level you add it as a map entity. Simular to a prefab but you can assign an action to it.

Hope that explained it for you.

Windows XP SP3
Intel Dual Core CPU: E5200 @ 2.5GHz
4.00GB DDR3 Ram
ASUS P5G41T-M LX
PCIE x16 GeForce GTS 450 1Gb
SB Audigy 4
Spyware Doctor with AntiVirus