Use a listview that renders the table rows. Add an extra row under each row, that is hidden in HTML.
style="display:none;"
In this row you can add more controls and with jquery you can make it visible when the user clicks the + button.
var textbox = $('#textboxid').css('display', 'block');
Another possiblitiy is to postback to the page when the user clicks the + button. After that determine what row is clicked, and only add the extra row for this clicked row.