There is a way to do this using one subscription plan for the subsites.
It involves manually editing your pages and testing for active subscriptions yourself.
All the subscription forms and links you will need to create yourself. they must all point back to the main site.
All purchases, users must be in the main site profile.
you need to be familiar with PHP and Mysql and understand the WP templates
Here is how I did it.
Instead of using restriction shortcodes, I created a custom field called premium_content.
In Single.php , test for premium content. If the user is not logged in, show the excerpt then the login form.
You can create a functions in the functions file to handle most of this.
If the user is logged in, call a function that executes a mysql statement that searches the subscriptions table. SQl Statement will look something like this.
SELECT * FROM wp_users INNER JOIN wp_pms_member_subscriptions ON wp_users.ID = wp_pms_member_subscriptions.user_id WHERE 1 AND user_id = '$userid'
if done correctly it will return all of the information about that users subscriptions.
From their you can show them all the content, if a paid user, or show them LINKS back to the main site. You can try creating a form for the instant purchase, or to resubscribe, but it easier to link the user back to the main site.
Remember Everything has to point back to the main site.
Don’t modify the plugin otherwise the next update will wipe out all your work.