In order to use separate methods of payment for different Droplets, these would need to be created under different accounts.
You can definitely transfer a snapshot of your Droplets between accounts. This post describes how you can transfer a snapshot of a Droplet to another account on DigitalOcean:
If you have any other questions, you can definitely reach out to the support team for a speedy response. DigitalOcean's support is available 24 hours a day, every single day of the year. They're more than happy to answer these types of questions or assist you in resolving technical issues.
Platform Support Specialist