hello @pillarcom yes it's possible. So much can be done on the Unaverse.
1.) The buyer must actually initiate the transaction as if he wants to subscribe, then seller should check is pending order @ https://yoursite.com/payment-orders?type=pending to approve.
2.) Another method is for seller to process as manual order @ https://yoursite.com/payment-orders. After client has made payment, click on "manual order", select the client, add order id (you can put any random characters or client payment no to serve as tran id), then select "paid level" and choose the membership item client paid for and submit to add.