Each payment method model supports a member called $_canUseInternal
. This decides if the payment method can be used in the backend.
So you need to add this in the payment method model
protected $_canUseInternal = true;
But chances are you will need more than that, depending on the payment method. You may need to create a block and/or a template for admin use.
Some of the payment method might not be designed to work on the backend.
[Edit]
If you need a method just to appear on the invoice and no actions behind it then you can create your own. Here is a nice tutorial for creating one.
Just make sure you set to it
protected $_canUseInternal = true;
protected $_canUseCheckout = false;
protected $_canUseForMultishipping = false;
so you can use it only for frontend.