Is it possible to service rear derailleur pulleys?
Yes, the pulleys of your rear derailleur can easily be removed, cleaned, greased, and re-lubricated. I'd recommend doing so about every 500 miles depending on your climate or whenever they start to sound gritty again.

If you haven't yet, I recommend reading all the Park Tool drivetrain care topics.

Steps 9 through 13 particularly on this page should cover the pulleys:

https://www.parktool.com/blog/repair-help/rear-derailleur-overhaul