Cheat Type: Secret - Non-obvious in-game tricks or hidden features to ease 
or enhance gameplay It's quite simple:
Go to My Documents\\My Games\\Age of Empires 3\\Savegame open the 
"sp_yourcityname_homecity.xml" 
(make sure you back up this file) and find the line [activetechs]
This are all the cards you can use in the game. 

For Example:
I copy my tag: [tech dbid =\"2030\"]HCExtensiveFortifications[/tech]
I paste this tag between the tags: [cards] [/cards]
After that, i change the tag [tech into  [card and [/tech] into [/card]
Save the file and open the game. Play a skirmish game and you'll notice 
that you have 21/20 cards! 

You can do this with every card you have aviable. The only problem we have 
is the fact that you can't have more than 11 different cards of age 1,2,3,4.
So you'll have to find out which cards are for what age.