Thinking more about this and assuming I understand what you want to do, it isn't clear that there is an automatic solution. The "built from other columns" idea won't work
I think what you want is the following:
- Search for hierarchical tag 2.x.y
- Send books to CC
- Some item in the grouping drawer now contains 2.x.y (and only 2.x.y)
- Search in calibre for 3.a.b
- Send books to CC
- The above-mentioned item now contains 2.x.y and 3.a.b.
In other words, there must be memory of what you have sent to CC in the past. Nothing in calibre supports that function automatically.
What you can do is create a custom column "Text, behaves like tags" to hold the genres of interest. This column would not be hierarchical. Lets call this column #cc_genres. Such a column could be used as follows:
- Search for hierarchical tag 2.x.y
- Select all the books found
- Use bulk metadata edit to add 2.x.y to #cc_genres
- Send books to CC
- #cc_genres in the grouping drawer now contains 2.x.y (and only 2.x.y)
- Search in calibre for 3.a.b
- Select all the books found
- Use bulk metadata edit to add 3.a.b to #cc_genres. #cc_genres now contains two items.
- Send books to CC
- #cc_genres item now contains 2.x.y and 3.a.b.
The number of items in the list will grow over time. Occasionally you would need to remove any items (such as 2.x.y) that you no longer want shown in CC's grouping drawer.