You can:Once the FULL SDK built on the new version of DUSK is available SOON™ you will be able to play all the custom maps with both Classic or HD visuals, replace weapons, models and more.
Make and play custom maps, (even basically drag and drop Quake and Half Life maps) use texture replacements and sound replacements. Plus every file from the base game is available for you to use.
You cannot:
Have custom models (enemies), entities, weapons, and nothing from the HD pack is available for you to use yet.