1. Download and install the mod which includes the race you wish to add (check that you can open and play that mod on it's own first).
2. Open the 'codex_3_6.module' using notepad (backup before hand). The codex_3_6.module is located in the Soulstorm directory (usually 'C:\Program Files\THQ\Dawn of War - Soulstorm').
3. On the line above 'RequiredMod.1 = DXP2', enter 'RequiredMod.1 =*mod.module file name*, for example 'RequiredMod.1 = BTmod'. All mods have a .module file located in the Soulstorm directory, just copy and paste this name in.
4. Rename 'RequiredMod.1 = DXP2' and 'RequiredMod.2 = W40k' to RequiredMod.2 = DXP2' and 'RequiredMod.3. = W40k'
5. Save the file.
6. To add more race mods, simply add extra lines in above DXP2 and W40K, but always make sure they are at the bottom with the highest numbers Eg. they would become 'RequiredMod.3' and 'RequiredMod.4' if another mod were added. .module file first.
Alternatively, you can just copy the codex.module file located in 'C:\Program Files\THQ\Dawn of War - Soulstorm'\codex' and replace the codex.module file located in 'C:\Program Files\THQ\Dawn of War - Soulstorm'. NOTE: This will only add Black Templars and Tyranids. These mods will need to be installed for this way, aswell.