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