Dark Eldar are a Pirate Fleet, so they would do the following things:
-Setup a Fleet list with 1500 points instead of 2000.
-Secretly choose one of the systems on the map, and write it down. This system is their Pirate Base. This counts as a Hive World for them.
-During Play, any Battle scenario involving the Dark Eldar is instead a "major raid" which rolls on the Raids mission table. The points value of the Major Raid is 750-1000 points instead of the normal 500-750 points.
-Because Pirate Fleets can only participate in Raids, they cannot permanently own systems, but can own them for one turn when winning an attack in a Raid scenario.
-Once any opponent of the Dark Eldar gains 21+ renown, they may challenge the Dark Eldar player and then roll a D6 and multiply the result by 10. If this number is less that the Renown of the Dark Eldar player, the Dark Eldar player must tell this other player which system their Pirate Base is in.
-Once the Pirate Base is discovered, if the base is in a system they control, they may choose to play a Planetary Assault or Exterminatus! mission. If they win, the Dark Eldar's Pirate Base is destroyed!
-There are no rules for replacing a base. The Campaign Arbitrator can make house rules. I suggest succeeding in a dedicated appeal two turns in a row to get a new secret base.
-If the Dark Eldar player is using a Haven station as their Pirate Base, and the base is discovered, but not destroyed, it may attempt to move through the webway. Roll a D6. On a 1 the Haven is destroyed, but on a 2 it becomes hidden and the Dark Eldar player can choose a system to hide it in.
Hope this helps!