I believe that what you are asking is whether any more characters in the novel Milkweed are inspired by actual people who survived the holocaust. You are also correct in that the character of Doctor Korczak is based on a real person named Janusz Korczak.
Jerry Spinelli, in an interview for TeacherVision.com, responded to the question of whether there are any other characters based on real people:
There are bits and moments of people I know ... in most of the characters, though none was inspired primarily by a particular acquaintance. Historically speaking, of course, there are the characters of Dr. Korczak and Himmler.
Along the interview, Spinelli explains that his purpose was to make his characters equally relatable whether they existed in real life or not. The Holocaust is one of the darkest periods in the history of mankind, and in Sprinelli's own words, "there is no statue of limitations on humanity".
The more the writer explores the traits and personalities of every individual that could have been a victim of that horrid event, the more we get to analyze our own nature, and understand our reality as members of a race that is capable of incredible highs and abysmal lows as well.