Ctrl–v enters visual mode blockwise. You can then move (hjkl-wise, as normal), and if you want to insert something on multiple lines, use Shift–i.
So for the text:
abc123abc def456def ghi789ghi
if you hit Ctrl–v with your cursor over the 1, hit j twice to go down two columns, then Shift–i,ESC , your text would look like this:
abc,123abc def,456def ghi,789ghi
(the multi-line insert has a little lag, and won’t render until AFTER you hit ESC).