You can create a profile file for each instance of DB2 and place environment variables in the profile file. You can use this profile to create unique for each database user. Each DB2 instance has a home directory that is associated with the instance's log on ID, for example:$InstHome/sqllib/db2profile
Place the variables that your are interested in using (for example the TimeZone variable) in the profile file. When you then issue a su -db2instX command (where X is the instance's name), the environment variables are sourced.