How do I add or modify environment variables for systemd service that does not already support setting via file in /etc/sysconfig/ directory?

Solution In Progress - Updated 2014-11-13T17:52:53+00:00 -

Issue

How do I add or modify environment variables for systemd service that does not already support setting via file in /etc/sysconfig/ directory?
How can I change PGDATA or PGPORT for postgresql when starting via systemd?
How do I customize a unit file/ add a custom unit file to systemd?

Red Hat Enterprise Linux (RHEL) 7

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