With the revelation of the cloning of Nathen, why not open the door to a clone of Admiral Dumar and Prince Cashmire
The cloning process requires a genetic sample from the individual being cloned and a complete consciousness scan to "resurrect" someone. Casimir died without such a scan and it isn't stated anywhere the such a scan was done on Dumar in time for it to matter since the scan needs to be done within two minutes of death. I suppose their bodies could be cloned, but without the consciousness scan they would be little more than comatose shells or empty vessels that would need to relearn everything they've learned in the course of their lives.
Jason is correct in that to resurrect either Casimir or Dumar, you would need their consciousness and memories to have been scanned within minutes of death. However, you could clone them from any DNA sample, even one obtained long after death.
They would, however, be empty slates, and would have to be raised naturally as infants through adulthood.
Clones grown on Nifelm are grown in acceleration vats that brings the clone body to maturity in as little as 12 months. (Normally, 24 months.) However, the clone body is kept in a comatose state, keeping the mind a "clean slate" into which the saved consciousness and memories of the source body can be transferred.
You could clone Casimir and Dumar and raise them naturally, but, if the intent is to get the original back, there would be no point.
Getting the scan of their consciousness would defiantly be the hard part. Unless Ryk decides to introduce time travel, very complicated at best, getting that information to the future would be impossible as two way communication would be required to make the scan.