It's a completly different system of level creating.
A lot of games do not have their own level editor, so they're using one of the major 3D apps. to build their levels from, e.g. Halo CE is using this method.
It also got a tool to make the level architecture playable by placing entities & creating scripts (so called tags).
I guess another way would be to use custom objects placed in the 3D app. to create entities.
Some kind of modified version of the 3D app. to suit the needs of the level designer.
However this method is as different from Hammers level creating concept as it can get.
I think i described a possibility how the converting could work in the other thread.
But seriously, do not just use ripped content to build a map. The result won't look good anyway.
Extreme lowres textures, no lightning >> causing everything to be fullbright, etc.