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.

Yes, that's exactly what I found. I listed the pricing in another
thread. Thanks.