We do not support converting a Droplet IP address into 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 IP addresses available for Droplets only. When a Droplet is destroyed, its IP address is released and moved back to the pool. It is not possible to get the same IP back from the pool because the IP addresses are assigned automatically and we cannot hold, reserve, or assign specific Droplet IP addresses.