I don't believe there is any external preset or style file which they are stored in. I think they're probably stored within the InDesign preferences.
If you wanted them used in all future InDesign files, you could define them with Type > Text Variables > Define when no documents are open, and they should appear in any new document. (I haven't tested this but it should work, I think.)
In the Text Variables dialog box, there is a Load command, so you should also be able to import them by selecting another InDesign file that has the text variable(s) you need.
What are you trying to do? Use existing variables, or define some new type of variable that is not yet implemented?
I wanted to use this script:
...to establish a 'last modified by [username]' variable for some of our documents here (along with a regular modified date one). but as it updates the variable on a document open event, it simply shows whatever user has the document open at the time, not so useful. so i was hunting through the other event types for something suitable. i thought the Modification Date variable must be doing something similar at the point i'd want the user's name variable to update, so i thought if maybe i could have a look in it i could apply whatever it's doing to the new variable. having it run 'beforeSave/As/Copy' seems to work alright though. 'before/afterClose' didn't seem to have the desired effect, though i would ideally like it to update whenever the Modification Date updates.