The method described below doesn't keep texture coordinates, patch meshes, models or entities, everything but map's core's geometry is lost in the process.
- Open the map you wish to convert in GTKradiant
- Hide or delete hint brushes, cluster portals, liquids, triggers, etc. Leaving just the map's solid structure. You can left out the sky too because leaks doesn't exist in UDK and UDK doesn't render portal skies or skyboxes
- Select all brushes (entities can be selected too, doesn't matter)
- Export to OBJ using brushexport2. To collapse or not the mesh doesn't matter
- Convert OBJ to FBX (Deep exploration trial can do it)
- Import in UDK's content browser
- The map's mesh should be rotated, rotate it to correct it
- In the newly added static mesh's properties uncheck Use Simple Box Collision, Use Simple Line Collision and Use Simple Rigid Body Collision. Leave only "Use Per Poly Collision" enabled.
- A single skylight should be enough to make whole map full bright
- Don't forget to change kill Z or move the mesh to prevent player from dieing instantly
Here is legodeck in GTKradiant flatshaded and wireframe:
Legodeck in UDK:
Notes: UDK can also import ASC, but for some reason UDK crashes when it tries to import a complex mesh such as a whole map converted to ASC. There is an utility called ASEtoT3D which could be used in conjuntion with q3map2's convert to ASE feature, but ASEtoT3D doesn't understand the ASE generated by q3map2 so this path doesn't work.