CBSubs doesn't have a Joomla metadata file yet so menu parameters aren't possible for it yet. It is possible to create Internal menu items to plugins now though. For example see the Plugin CB menu type and then select CB Blogs and you should see its menu parameters. We've plans to extend this usage to all our plugins with frontend pages.
Actually this is a pretty simple addition. Add the attached file to the below location (unzip first).
components/com_comprofiler/plugin/user/plug_cbpaidsubscriptions/xml/metadata.xml
Next create a new Joomla menu item with the type of Community Builder > Plugin. Next select "CB Paid Subscriptions" for "Plugin" then just supply the plan ids to "Plan" as dash separated (just like you would when using the URL externally) and you'll now have an Internal Joomla menu link to CBSubs Plans pages.
Is there a way of displaying a single plan in a Joomla article? or in a module?
No, but you could add a link in your article to the plan.