Brush Exporting And Importing

An exported .t3d brush file is also called a prefab.

Exporting a Brush

Create the shape you want exported in the world. This means actually make it out of "matter".
Get the texturing on your object the way you want it.
Intersect the object you want to make a prefab.
Do UnrealEd Main Menu → Brush → Export
Choose a directory to save the .t3d file to and export it.

Importing a Brush

Do UnrealEd Main Menu → Brush → Import
Navigate to the .t3d file you want to import, select it, and click open.
Click merge faces if you want the editor to merge co-planar faces. If you don't know what this means, leave it unchecked, because you can always do this later yourself once it's added by doing Brush Context Menu → Polys → Merge.
I always leave the radio buttons alone since I don't understand what they do but perhaps someone who does could fill this in... Bean: It's best to not merge faces and to change it to a nonsolid using the radio buttons. This reduces most BSP problems right off the bat. If those options cause problems, you could always play around with others.
Click OK, your red brush is now the shape of your prefab and will have the same textures when you add it as it did when the prefab was created.

Notes

Re-Importing Errors

One note I'll add is be careful saving your cherished brush masterpiece by exporting it. UnrealEd's import process does not look at the the "Normal" attribute for polygons in the .t3d file during its import process. Rather, it recalculates them. If brushes are too complex or step on UnrealEd's toes in the wrong way (I've never nailed down exactly what), you'll get a bad normal calculation on some surfaces and they will refuse to take lighting properly... from then and forever and ever... amen.

If you have uber cool and complex brushes that you must keep, put them in a warehouse (a big subtracted cube somewhere off to the side) in your map.

Brush Save