This Segment Destination Function enables taking a standard user-trait and "promotes" it to an external-id. This function can be connected to any source (including, for example, as a destination to a SQL trait).
To install, use Sloth Package Manager simply run (if new to spm, be sure to first run spm init):
spm install christyharagan/sloth-github-action
Then in your Segment workspace create a new destination from the function called "promote_to_externalId"