Rewrite you can do this magento two process

Process One:

one is rewrite the class Mage_Checkout_CartController and in this addAction() at code if (!$cart->getQuote()->getHasError()) { add redirection code $this->_redirect('checkout/onepage');

Process two:

Using event/Observer

Event:

checkout_cart_add_product_complete

This event provide the data of product object and cart request parameters

Mage::dispatchEvent('checkout_cart_add_product_complete',
    array('product' => $product, 'request' => $this->getRequest(), 'response' => $this->getResponse())
);

Config.xml code:

<frontend>
    <events>
    <checkout_cart_add_product_complete>
             <observers>
             <packagename_modulename_observer>
                  <type>singleton</type>
                  <class>packagename_modulename/observer</class>
                  <method>addToCartComplete</method>
             </packagename_modulename_observer>
             </observers>
         </checkout_cart_add_product_complete>
     </events>
</frontend>

Observer code:

using this event you can set your return url

$observer->getEvent()->getRequest()->setParam('return_url',Mage::getUrl('checkout/onepage')); this redirect to you at checkout Onepage function:

public function addToCartComplete(Varien_Event_Observer $observer) {
 $product = $observer->getEvent()->getProduct();
 $Request=$observer->getEvent()->getRequest();
 $Request->setParam('return_url',Mage::getUrl('checkout/onepage'));
}

this redirect to you at Checkout onepage

The add to cart request is a GET request that among other params, it can contain a uenc param with can hold a base64 encoded return URL. The user is redirected to that URL upon return from add to cart.

You can simply create a new button that you add anywhere on the page, that on click will do a GET request to a URL like https://example.com/checkout/cart/add/product/XYZ/uenc/aHR0cHM6Ly9leGFtcGxlLmNvbS9jaGVja291dC9vbmVwYWdlCg==/ where aHR0cHM6Ly9leGFtcGxlLmNvbS9jaGVja291dC9vbmVwYWdlCg== is base64 encoded https://example.com/checkout/onepage