Rename the new sidebar file to sidebar-center.php and then use <?php get_sidebar('center'); ?>
immediately above (or below) the call to your existing sidebar.
In terms of registering the new sidebar, have a look what is being used for your existing sidebar in functions.php and base your new register_sidebar()
on that. Mine tend to go something like:
FUNCTIONS.PHP
if (function_exists('register_sidebar')) {
register_sidebar(array(
'name'=> 'Main Sidebar',
'id' => 'main_sidebar',
'before_widget' => '<li id="%1$s">',
'after_widget' => '</li>',
'before_title' => '<h3>',
'after_title' => '</h3>',
));
register_sidebar(array(
'name'=> 'Center Sidebar',
'id' => 'center_sidebar',
'before_widget' => '<li id="%1$s">',
'after_widget' => '</li>',
'before_title' => '<h3>',
'after_title' => '</h3>',
));
}
SIDEBAR.PHP
<div class="sidebar" id="main_sidebar">
<?php if (!function_exists('dynamic_sidebar') || !dynamic_sidebar('Main Sidebar')) : ?>
[ hardcoded stuff ]
<?php endif; ?>
</div>
SIDEBAR-CENTER.PHP
<div class="sidebar" id="center_sidebar">
<?php if (!function_exists('dynamic_sidebar') || !dynamic_sidebar('Center Sidebar')) : ?>
[ hardcoded stuff ]
<?php endif; ?>
</div>
Giving each sidebar its own unique id means that you can then apply specialised CSS as needed whilst still styling generic stuff using the .sidebar class.
Hope that helps