Whilst technically not board appropriate the following ways of including these armies is to allow players to use them in the thirty first millenium.
You may post pictures of Necron Tomb Guardians, stranded Tyranid forces (not hive fleets) and xenos represented by the Tau Codex (althought they must be converted to look distinct to Tau) in the Enemies of the Imperium painting and converting forum. Please try to be as creative as possible though to avoid the thread being removed/closed.


The Necrons

The Necrons are a terrible and ancient race that has long become dormant since it went into statis, waiting for the time that the C'tan will once again wake to feast on the souls of the living.
Technically speaking the Necrons aren't really what you might concider a race of the thirthieth millenia. However, there is no reason why you can not field a Necron army in this time setting if you should so wish.
With no fore knowledge of the Necrons these sinister alien armies would represent unknown nightmarish attackers spilling forth like an army of ants from a disturbed hive only to return to their slumber once the invaders have been dealt with.
The history of the Imperium spans ten thousand years, with much of the official records lost to war or internal conflict it is plausible for many incidents involving what are seen as minor races being lost.

The Necron Tomb Guardians

Therefore Necrons can be used as defenders of a tombworld but with the following restrictions. The following units may not be used from Codex Necrons to represent a Necron Tomb Guardian army:

C'tan
Pariahs
Monoliths

These units are left out of the list because the army represents the firces that are awoken to defend the Tomb. C'tan are still asleep, Pariahs haven't been created yet and Monoliths would take too long to activate.
The army represents a defence force and the Necrons are not yet space bound.



Tyranids

Tyranids are another old race, although an old race form a distant galaxy. Hive Fleet Behemoth are then first known Hive Fleet to attack the galaxy but it is possible that advance scouts made the perilous journey millenia before them.
Therefore it is possible to use an army of stable gene-stock to represent an advanced guard much like the Genestealers that have become trapped and isolated on a world without the means to escape.
Tyranids can be used with Codex Tyranids but for the purposes of campaigns, like the Necrons, they are never the invaders of worlds.
If anyone is involved in battle with them it is because they have inadvertantly stumbled onto one of their worlds and accidently stirred up a hornet's nest.



Tau
Tau are an extreme longshot, having only been around for approximately two thousand years in the forty first millenium.
Tau could be used to represent an alien force/raiders or like suggested in the Renegade Humans suggestions list used to represent advance humans.