If you're interested (in single-player mode) in stealing resources from other empires, try the following method:

• 1.) Start a new game and play as usual, giving the other empires time to build up their resources.
• 2. When you're ready to steal resources (as you're running low on resources yourself), save your game, quit, and then reload another empire to play as.
• 3.) Once you have loaded your alternate empire, go to the Diplomacy menu. Send the resources from this empire to your "normal", default empire.
• 4.) Save the game, quit, and reload your original empire. The resources sent from the alternate empire should now be there -- you can repeat this as much as you want.