product type – external – "get_product_url" instead "permalink"

hi,

i want to change the “read more” links in the shop overview with the direct link to the external products.
in the file “add-to-cart.php” i found the right code to replace, but my tries faild to change this …
i think, maybe it could go with the code in this file “grouped.php” (on line: 41).

how can i do this, can anyone help me in this?
would be nice.

http://wordpress.org/extend/plugins/woocommerce/