The 'recurring subscription' within Momoyoga is meant for the 'tacit renewal' of a monthly subscription. This is often used for yogis who are weekly booked to a fixed class of a series that never expires.
A credit is usually used for flexible booking and cancellation of classes. It ensures a yogi is not attached to anything. When a credit has expired, a yogi does not have to purchase a new one. This makes it difficult to give a yogi a permanent place, for a series of classes that never ends. After all, you are not sure whether a yogi will buy a next credit and comes to class.
If you still would like to add a yogi to a series of classes with a credit, this is possible. You will notice from some point the yogi is signed into this series of classes without payment. The credits or date from the class pass has been finished. You can check if the yogi has purchased a new class pass, or you can ask if he would like to purchase one.
After the yogi has purchased a new credit you can delete the yogi from the first class without payment from these series and directly re- add the yogi. You will see the yogi is now signed in with payment again.