This is quite an unusual numbering - may I ask why you want this kind of numbering?
And also it is not clear to me what should happen when you insert a new row.
Should it then be like this?
1. one | a. dos | 4. tres | d. anothernewone
2. two | b. three | 5. quatro | e. yetanotherone
3. uno | c. four | 6. newone | f. anotherone
Not sure whether you’re talking about a table or “text-flow-columns” either.
Adding a “row” sound like you’re talking about a table, but even then I cannot think of a case where this kind of numbering would make sense…
So while the “number range” fields can be used in a table and when the numbering is increasing horizontally (1… a… 2… b…) it won’t work vertically. The term “adding a row” doesn’t really fit with text-flow-columns concept, so I’m quite puzzled by your question…
Even with helper-rows/columns to get a count, you’d still need to manually adjust the references after adding rows.
Please provide more background info on what sense is behind this numbering. Maybe then people can come up with better suggestions.