Assign Stripe Customer to Memberstack Subscription in Stripe

We are attempting to migrate customers from another service INTO memberstack. 

It looks like we can assign customers to the Memberstack generated product within Stripe's admin. 

However if we were to import these customers into Memberstack via manual or CSV import we cannot assign them to the paid subscription. 

Is there any way to do this? Manual or otherwise, it'd be a death sentence for us if we asked customers to re-enter credit card information.