WordPress Multisite allow site admin to add user without email confirmation

You will not be able to enable the checkbox for non-super-admins as the code for it shows:

<?php if ( is_multisite() && is_super_admin() ) { ?>
<tr>
    <th scope="row"><label for="noconfirmation"><?php _e('Skip Confirmation Email') ?></label></th>
    <td><label for="noconfirmation"><input type="checkbox" name="noconfirmation" id="noconfirmation" value="1" <?php checked( $new_user_ignore_pass ); ?> /> <?php _e( 'Add the user without sending them a confirmation email.' ); ?></label></td>
</tr>
<?php } ?>

You can disable the sending of the confirmation emails altogether with the following:

add_filter( 'wpmu_welcome_user_notification', '__return_false');

The other option is create a plugin that makes your own custom add user page.