Kantu cannot modify csv, it can only append lines (with columns, but still new row is mandatory).
There is a crippled workaround and not that much crippled.
A) You could add new row with url that you’ve read and number in the next column. But it’s crippled because you won’t be able to read it again in kantu. Weird? I know. But if you won’t be reading it again and ok with two lines then go for it.
B) You can save new csv with other name with one row and two columns.