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.
Bottom line:
Please provide more background info on what sense is behind this numbering. Maybe then people can come up with better suggestions.