If anyone is interested here is the code to do this.
Add Configure now button to special products instructions:
Please edit the '<xcart-dir>/skin1/customer/main/buy_now.tpl' template and insert the lines
{if $product.productid eq 1 or $product.productid eq 19 or $product.productid eq 20}
Configure Now {/if}
directly before the
{include file="buttons/buy_now.tpl"}
Please substitute the PRODUCTID1, PRODUCTID19, PRODUCTID20 with actual productids of the products for which you would like to add the button.