Karuna Reiki can be learned by any one who is already practicing Usui Reiki. There are 3 levels in Karuna Reiki.

Karuna Reiki level 1

This level is given to those who are already practicing with second level of Usui Reiki. In this level attunement is given to activate first four symbols of Karuna Reiki.
This level is done in 3 days workshop.

Karuna Reiki level 2

This is level is given to those who are practicing with the third level of Usui Reiki or some call it 3 A. In this level attunement is given to activate next set of four symbols.
This level is done in 4 days workshop.

Karuna Reiki level 3 or Master level

This level is taught to those who are experienced Reiki Master or 4th level or as some call it 3 B. If a person wants to do Karuna Reiki Masters, it is not necessary for them to do 1 and 2 level of Karuna Reiki, they can directly go for Karuna Reiki Master level. But if they want to go through all the levels before Master level, it is fine. In this level attunement is given to activate four Master symbols.
This level is done in 7 days workshop.