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 cat /etc/passwd
The following is an example of an /etc/passwd file:
root:!:0:0::/:/usr/bin/ksh
daemon:!:1:1::/etc:
bin:!:2:2::/bin:
sys:!:3:3::/usr/sys:
adm:!:4:4::/var/adm:
uucp:!:5:5::/usr/lib/uucp:
guest:!:100:100::/home/guest:
nobody:!:4294967294:4294967294::/:
lpd:!:9:4294967294::/:
lp:*:11:11::/var/spool/lp:/bin/false
invscout:*:200:1::/var/adm/invscout:/usr/bin/ksh
nuucp:*:6:5:uucp login user:/var/spool/uucppublic:/usr/sbin/uucp/uucico
paul:!:1000:1000::/home/paul:/usr/bin/bash
jdoe:*:1001:1001:John Doe:/home/jdoe:/usr/bin/bash
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.