What you’re looking for here is coming, but isn’t possible yet in Roon.
You can currently edit the display of many different fields, such as changing the name of a track or composer.
The ability to edit the links between the “objects” in Roon’s database (such as the link between the track Domino, and the composers Van Morrison and Gene Simmons) is coming in the next round of editing functionality.
I discussed this in slightly more detail the other day.