Generally a straight conversion is not possible as the resource types (especially materials and sounds) are way to different.
You can try looking for a tool that converts the T3D file of an exported UE2 map into the format the UE3 editor expects for importing. Don't expect any actors to be converted correctly. Also note that you need to import materials, meshes, sounds, etc. before importing the map, otherwise import may fail or references to missing resources are cleared.