It is easier if the folder has not yet been imported:
Add the Pitchfork folder.
Go to the Album browser.
In Focus, click on the Added diagram; in the pop-up, click on Added in the last day.
Select All.
Manage Tags, add Pitchfork 500
If the albums are already in the library, you might be able to do this, I’m not sure.
In Windows, create a Temp directory next to the Roon music directory, add it as a watched folder in Roon
Drag the Pitchfork directory from the Roon directory to Temp
Focus on storage location Temp
Select All, manage tags, add Pitchfork 500
Drag it back to the Roon folder.
I’m pretty sure that Roon will recognize that this is the same content as you move it back and forth, so the tags will persist.
And note that the movements take no time, as long as the two directories are on the same disk, next to each other. No data is moved, it is just changes to tables.