By giving your account login details to your teammate for the whole game.
By gifting a territory to you teammate for individual territories.
By airlifting armies from your own territories to your teammate if you want your teammate to have your armies.
By transferring your armies from one of your own territories to one of your teammate's territories.
No other ways exist unless you put this idea on Uservoice or search for a similar idea like this on Uservoice and up vote it. Link to Uservoice: http://warlight.uservoice.com/