roon cannot do this currently.
There are several requests for this and the team behind roon is aware of this. No idea what the status is progress wise.
This is not as easy as you would think. As example, Is there artwork / full metadata available for each individual disc that roon can pull?
I my opinion the boxset handling is not on the same level of sophistication as the rest of the metadata capabilities in roon. Especially the disc numbering is the weak point. Hope they can improve on this.