De um artigo cyberciti.biz :
Following files/dirs are required for traditional Linux user management:
/etc/passwd - contains various pieces of information for each user account
/etc/shadow - contains the encrypted password information for user's accounts and optional the password aging information.
/etc/group - defines the groups to which users belong
/etc/gshadow - group shadow file (contains the encrypted password for group)
/var/spool/mail - Generally user emails are stored here.
/home - All Users data is stored here.
You need to backup all of the above files and directories from old server to new Linux server.