In order to use the combined membership option and add a membership as a free bonus you would need to use the combined membership feature to bundle it. The membership would need to be set to free or free trial to do that.
There are several strategies you can use with coupons as well, like giving a coupon to get the membership free or delivering bonus content to your main offer if your clients use a coupon with episodes assigned to it, etc. If you have a current membership that is paid and you want to give it free you can mirror it and set the price for free and disable the new mirrored version from allowing affiliates.
The main thing we recommend is to make all your offers memberships... even ebooks. It gives you access to more features like bundling, etc.