Login to the account you wish to add a subscription to and click this link
Click the add package button.
Select the subscription package, add the channels you wish to include in your subscription and click continue.
Confirm the payment method you wish to use and click confirm and pay.
Please note: Your payment method will be charged for the remaining days of the current billing period, this is to allow you to use music on your newly add channels straight away and to be charged for one payment a month going forward.
There you have it, you should now have successfully added additional channels to your subscription.