Since you want to emit multiple rows, you should be building one form element per row in your sources multirow.
ad_form -extend lets you do this. You'll need to number the checkboxes ... source_id_$i something like that?
Then perhaps you could write a custom widget that build the HTML for one row, and in your template begin and end the table that holds the widget rows.
Something along these lines, perhaps? Just thinking off the top of my head...
