These pages are not complete and I have not, as yet, provided detailed statistics.  I am more interested in providing suggestions and background information for now.  As you can imagine transcribing the related Codex into dnd speak would take some time.  This whole area is a work in progress and is mostly background information with little statistical info.  The Dark Eldar are the most complete.

Chaos Space Marines                                                                                                               From within the Eye of Terror and other dark sanctuaries of evil, the vile legions of chaos spill forth in a frenzy of wanton bloodletting and desecration.  Combining the armour and training of Space Marines with the battle-hunger and ruthlessness of Chaos followers, the Chaos Space Marines can inflict horrendous casualties on their enemies whilst sustaining little damage themselves.  The Chaos Legions often summon horrifying daemons to the battlefield.  These immensely powerful beings are the equal of an armoured vehicle or several squads of men, and can tear the heart out of an enemy with a single well-timed assault.  They are the sworn enemies of the Space Marines and all of humanity.  The major practical difference between a Chaos Marine and a Space Marine is that Chaos Marines use TL 12~13 gear (Space Marines are TL 14/15).  They may also take any "magical" feat or character class listed in the dnd 3e Players Handbook.  They are always evil.

Eldar                                                                                                                                                  The Eldar are the most mysterious race of all. Their race has suffered the most at the hands of Chaos, predominantly by the power of Slaanesh. Eldar are incredibly complex people with a way of life totally unique to themselves.  Use the statistics of Elves found in the 3e dnd Players Handbook. 

Eldar, Dark                                                                                                                                    Much like their cousins, the Eldar, the Dark Eldar are an old and advanced race.  In fact they are the same race as the Eldar.  But unlike the Eldar who have foreseen the future and accepted their fate (eventual extinction at the hands of man) the Dark Eldar war against fate, and in so doing have become twisted and hateful. For Dark Eldar use the Dark Elf subtype found in 3e dnd Monster Manual I.

Tyranids                                                                                                                                           Totally inhuman, the Tyranids are a deadly race of alien monstrosities that lives to consume all living matter. Encased in chitinous armor and wielding weapons that are an amalgam of living tissue and synthetic material, Tyranid broods rip, tear, slash, maim and mutilate everything in their path, their killing frenzy leaving nothing but dismembered corpses in their wake.  Tyranid hybrids use the SS Insectile template from SS. 

Orks                                                                                                                                                 Orks are greenskinned monstrosities who spread across the universe in vast and brutal hordes. Living to fight anyone they encounter and when they're alone fighting each other, Ork WAAAGH!s (as their barbaric warbands are called) drift across the galaxy spreading conflict and strife in their wake.  Ork weaponry and war machines are crude but effective and more than one more "sophisticated" army has fallen before the massed might of this savage green tide!

You can use the statistics for Orcs in the Monster Manual I or Half-Orcs in the Players Handbook for w40k Orks.

Necrons                                                                                                                                                 The Necrons are a biologically dead race that was wiped out in a galaxy wide catastrophe approximately 60 million years ago.  Faced with extinction the Necrons constructed metal bodies as repositories for their hyper-advanced minds.  To weather the impending galactic bio-meltdown the Necrons constructed huge stasis chambers to shelter inside.  They have been there ever since, resting until such time that the galaxy was safe for them to emerge.    

The Tau                                                                                                                                           The alien race known as the Tau inhabit an area of space near the spinward fringe of the Imperium and are a young, dynamic race, with highly advanced weaponry and technology. Though less than two thousand years old, their fledgling empire is rapidly expanding into space and encountering all the elder races of the galaxy.