I've been modeling and mapping for the Source engine and using SketchUp for years and wanted to go a step further after the Source engine hasn't met my expectations for a long time. Now I'm looking into UDK.
Unfortunately the start I had with UDK was pretty rough. After three days of trying out countless file formats, export plugins and converters (including HardPCM's export plugins), I still haven't found a way to get models into UDK that have a custom collision mesh as well as working smoothing groups.
With many file formats SketchUp (or the export plugins) not only added empty groups/meshes to the exported file but also changed or replaced the group names. Therefore I have not been able to create any collision meshes with the UCX prefix, even when I corrected the group names in the exported files (when the files were editable with text editors). UDK simply interpreted them as generic meshes. If only there was a way to import collision meshes (and additional UV maps) from other files ...
Only one plugin, an ASE exporter made for RollerCoaster Tycoon 3 modding, kept the names of the groups and exported the correct amount of groups. However these ASE files were also the only file format where the UDK decided to smooth every corner of the model which makes it useless.
So, is there really no way to get models from SketchUp to UDK with no loss of valuable geometry information?