As a subscriber, can I add a product to my susbcription?

Yes!

From your Ongoing Subscriber Portal > Click "Add Product"

This item will be added as a subscription to your existing subscription order.

Changes will be applied to your upcoming subscription order.

You'll just need to choose the product you want - choose a variant option and agree to the terms of the service.

No checkout needed - the calculations are instantly made for you and the new total price of your order will be adjusted accordingly.

You can always remove the item later if you wish, by clicking Remove.