In order to modify default environment PATH for ordinary users and superuser, edit file /etc/default/login as root user, and place your additions in the line PATH=.
You may have noticed that by switching user with su command, the environment PATH variable is reset to what seems like a default value. To alter this behaviour, edit file /etc/default/su and find the following lines:
PATH=is the value for switching toward an ordinary (non-root) user.SUPATH=is the value for switching to the root user.