I don’t think you can do that, however you can implement this code in your functions file to at least allow for line breaks in single image captions
Note: doesn’t work for gallery images (I am trying to find a way to make it work in galleries)
/*
*********************************************
Function to allow multi-line photo captions.
This function will split captions onto multiple lines if it detects
a "|" (pipe) symbol.
**********************************************
*/
/* Override existing caption shortcode handlers with our own */
add_shortcode('wp_caption', 'multiline_caption');
add_shortcode('caption', 'multiline_caption');
/* Our new function */
function multiline_caption($attr, $content = null) {
extract(shortcode_atts(array(
'id' => '',
'align' => 'alignnone',
'width' => '',
'caption' => ''
), $attr));
if ( 1 > (int) $width || empty($caption) )
return $content;
if ( $id ) $id = 'id="' . esc_attr($id) . '" ';
$new_caption = str_replace("|", "<br />", $caption);
return '<div ' . $id . 'class="wp-caption ' . esc_attr($align) . '" style="width: ' . (10 + (int) $width) . 'px">'
. do_shortcode( $content ) . '<p class="wp-caption-text">' . $new_caption . '</p></div>'; }
Original code by Mark Wood:
http://digitalnotions.net/multi-line-captions-in-wordpress