Adding a new user will not overwrite the existing /home/username. You will however, want to look at your current /etc/passwd file and make sure that the UID's for each user on the newly installed system match the existing one. For example, User 1's UID = 1000; User 2's UID = 1001; etc.
The way to check this is to open a terminal and run
The following is an example of an /etc/passwd file:
nuucp:*:6:5:uucp login user:/var/spool/uucppublic:/usr/sbin/uucp/uucico
So when you make new users be sure that Paul is user 1000 and John Doe is user 1001; etc. That way the files that belonged to these users will still belong to them. Linux identifies the file/directory ownership by UID's and GID's. So you'll want them to match.