Can I manually add a card number as mandate?
No, this is not possible via RCUR nor Mollie. Customer has to enter this by themselves to give approval. This can be done by creating a payment link in RCUR and send this to the customer.
How to create a mandate payment link?
Go to payment links and click on “Create payment link”.
As type, select “Mandate”

Fill in a description of choice, which is also visible for the customer
Enter an amount with a minimum of €0,01
Select the desired templates
Optional; Disable payment methods you not want to use by click “Payment methods” and toggling the unwanted methods.

Click “Create payment link” and copy the link in the green success message. Or click on the newly created link on the payment links page to open it in your browser. You can send this payment link to your customer.
After a successful payment, the credit card number will be stored as mandate (if paid via the cards payment method)