I have chrome shelving kit pipes with 3/8" threads that I'd like to
use for inserting casters instead of the levelers it comes with. I
have some 1/4" thread casters lying around and would like to find a
way to use them with the 3/8" threading. Is there such a thing as a
3/8" to 1/4" threaded reducer for casters? TIA.

You might find a "thread insert" somewhere that does it.

You would find the pitch incorrect then. Far better to just get some
correct size, not to mention load capacity, 3/8" castors and wait until
you have an appropriate use for the 1/4" castors

I found thread inserts to be so cheap that it would have been worth the
risk to see if they'd fit. But as you suggested, I ended up buying four
3/8" casters.