Combine Children In-Editor?

Is it at all possible to combine the child meshes/materials of a GameObject in the editor, rather than at runtime? I'd like to assemble level design "building blocks" into things like rooms and hallways, and then combine them into prefabs.

Sure can:

http://forum.unity3d.com/threads/37721-Combine-Children-Extented-(sources-to-share)

Hi!

If it is an option, you can use the Easy Mesh Combiner tool. It can combine your meshes quickly and very easily, 100% through the editor. It’s a wonder to combine the meshes of the scenery and even get more performance! You simply select which GameObjects or children that should have the meshes combined, right-click and combine everything!

Worth a try. Here is the tool link (Available in the Asset Store)