There is no possible way to use a Droplet IP as a Reserved IP. These two IP types use different pools of IP addresses that are not interchangeable.
Droplet IP addresses are assigned from a pool of IPs available for Droplets only. When a Droplet is destroyed, its IPs are released and moved back to the pool. It is not possible to get the same IP back from the pool, as the IP addresses are assigned automatically and we cannot hold, reserve, or assign specific Droplet IPs.
If you anticipate needing to move an IP address between Droplets, please use our Reserved IP feature. Reserved IPs allow you to add an additional, portable IP address to your Droplet, and move that IP to a different Droplet at any time.