I knew that'd be coming. 🙂
Yes, you can. How easy or difficult it would be would depend on the complexity of the page, of course, and whether you're using css or straight html, etc. But, simply, say you want to insert it into the first column of the first row of the first table, and you had a plain "<td>" tag there:
$td_pos = strpos($a_str, '<td>'); // Where "<td>" starts
$insert_pos = $td_pos + strlen('<td>'); // Where to insert
$a_first = substr($a_str, 0, $td_pos)
$a_second = substr($a_str, $insert_pos) // Split in two
$final_str = $a_first . $b_str . $a_second; // Join all
// Or
$final_str = str_replace('<td>', '<td>' . $b_str, $a_str, 1);