Here I will be using jaggery server to demonstrate this scenario.
Below is the Flow that I will follow.
Carbon Admin Services --> SOAP WS calls --> jaggery Module --> JSON --> jaggery App --> REST ful API for wso2 Carbon Admin Services.
Here are citations to some posts that can help you to achieve this.
To find out the correct message (SOAP Payload) that is exchanged in Carbon BE for Carbon, see this article. SOAP UI is not working for some wsdl in here. So better it's to try this way. 
Learn how to add a module for Jaggery 
Then you have to write a jaggery app using this module, that APP will have REST full support.
Then you can call
https://localhost:9443/Carbon/UserAdmin/listUsers (GET http request) will give json array with carbon USER list.
Here is figure of component and messages that will pass over system .
Normal Carbon platform
Here is RESful carbon With jaggery
Code level description will be in my next post.