Burley doesn’t produce a wheelset for its trailers that runs Presta valves (Burley, if you’re reading this… take note), and I didn’t fancy rebuilding the hub around a new 20” rim (although I’ll probably eventually do this, because the spoke nipples are developing a tenacious rust anyway…) drilled for Presta valves. So, I learned about ‘valve grommets’. Essentially this is a small… well, grommet... you insert into the Schrader rim hole, converting it into a Presta rim hole.
Why do you need this? If you simply insert a Presta valve...