Contact our team and we'll be happy to assist with an addition to your existing subscription. Please note, additions can be made prior to the next date showing on your product settings, however they cannot be made to confirmed orders (where you receive a confirmation and unique 'AT-XXX' reference).
Currently we are working on a way to update our subscription portal to allow customers to add their own additions.
Alternatively, if you have never subscribed before you can place a new order via our website and choose the 'subscribe' option, which appears on the product page: