Getting on board would be surprisingly easy, for the same reason that Null don't have missiles - they've never had to worry about ship-to-ship combat. Their ships are essentially transport and information-gathering vessels, rather than warships, so there are no weapons on the outsides. It's possible to approach them very closely in relative safety, though they will see a vessel (or flying person) approaching. During the attack, the small ships will regularly lower themselves into the atmosphere to drop Null troops, so going into space isn't necessary if an attack is timed right. They could be accessed via some kind of teleportation (including Fear's) or flight plus an explosion (or something else!) to get past the hull or airlock. The larger ships will stay in orbit above the city, and can only be accessed via teleportation or some kind of ability that allows survival in space.

On board a small ship they would find 15-25 Null troops, supplies, and weapons, all being transported to the battlefield. On board a large ship, they would find 50-70 Null troops (some in the process of being repaired or built), extensive weapon stores, a huge stockpile of parts and materials for repair, and an area specifically dedicated to repair and creation of more Null troops. Both types of ship would have computers and many sensors dedicated to tracking movements on the planet.