This is very doable by using CBSubs Content integration provided in the CBSubs package. Once you've created Plans and have installed CBSubs Content integration you may navigate to the Plan followed by Integrations tab then Extensions, which you can then use By URL Part to protect parts of URLs. See examples below for Kunena URL protection:
index.php?option=com_kunena&func=post&do=reply&catid=CATEGORY_ID
This will protect users from posting new forum threads unless they have a subscription to the plan.