Open the [game folder]\Data\world\maps\campaign\imperial_campaign\ descr_strat.txt file with Notepad. You will see a list of the nations you can use in the beginning, the unlockable ones and the non-playables. Cut and paste the unlockable factions to the playable nations. Save the file. Now go back to the game and complete a campaign mode and all of the factions will be unlocked. Do not copy the end non-playables.

Note: first make a backup before you start.