How does Shiro get into the Deadman Wonderland?
Even though Shiro and Ganta were childhood friends, 


  Shiro was used in experiments by the director (Hagire Rinichiro) and Ganta's mother. This was all before Deadman Wonderland was created, and when the director founded it, he created a special room for Shiro. Shiro wasn't imprisoned there, but lived there so that the experiments to create the "Wretched Egg" (Shiro's other personality) can continue, thus she was brought there by the director when Deadman Wonderland was created.


This was mentioned briefly in the Deadman Wonderland wiki.