You certainly can!
Login at client.paypaplane.com to begin.
Click on the Agreement you’d like to add a Payment Method to, then click the edit pencil next to the heading “Payment Method” and select the relevant option. Please ensure you select the correct card type if using card (Visa, MasterCard, etc).
Enter your details accordingly (please ensure details are correct to avoid failed payments and resulting fees!).
Once added, you can view all exisiting Payment Methods by clicking the small menu button (bottom right hand corner) then clicking “Payment Methods”.
This will show you all your Payment Methods, allowing you to switch back and forth between them as much as you like!