The threading on most barrel adjusters found on brake calipers are a
standard size -- Campagnolo barrel adjusters have a FLAT surface on the shaft and the receiving hole is not threaded -- but, to replace the barrel adjusters with another set means that you have to remove the old ones ... if you can remove the old barrel adjusters, then they will essentially be functional ...
Inline barrel adjuster generally have a smaller size thread.
While removing the calipers from the bike AND brake pads from the calipers and then soaking them in some light oil is the best option ...
If you aren't in a hurry OR you don't feel like removing the brake calipers from the frame-and-fork, then you can simply seep a drop-or-two of regular "household" oil (e.g., 3-in-1) onto the threads ... come back in 24 hours ... test ... wipe off any excess & repeat until loosened enough to turn-and-adjust.
BTW. If your bike has clearance for fenders, then you can replace the brakes with some
LONG REACH brake calipers (
formally known as "normal" reach -- generally, a reach of 49mm-59mm).