Modifying Custom Table Style - Custom Table Elements?

I'm trying to create a custom table format. Toward the end of the table, there are a series of columns grouped together by a common theme. To make this visually clear that these columns are thematically distinct from the rest of the table, I want to create a solid singular border starting about 4 columns away from the end of the table that grows downward as you add more data/rows to the table. I'm not sure there is a way to do this but was wondering if anyone knew of a way. From what I can tell, the 'Modify Table Style' box doesn't permit custom table elements. I've attached an example of what I'd ideally like it to look like.