Hello Johan
If I understand your requirement correctly, it's possible. In this case, to simplify process for administrator, I think you can use offline payment plugin
membershipprodoc.joomservices.com/basic-...gins#offline-payment
Basically, people can still subscribe for your subscription plan, enter all the information you want to collect from these users and choose offline payment for their subscription.
These subscriptions will remain Pending. Then users can send payment to you somehow. Once you received payment, you can change status from Pending to Active to allow these users to access to the restricted articles on your site
I think that would be the best workflow for your case. If course, if you want, admin can create these subscription records manually from backend (or import these subscriptions from a csv /Excel file) if he wants
Tuan