That's going to take some source code customization to accomplish if you're going to use the built-in 'Add to Wishlist' button. What you could do instead is set the ShowBuyButton attribute to false, and then in the product description you can add your own button, with a link like this:
Code:
http://www.yoursite.com/addtocart.aspx?productid=#&quantity=#&iswishlist=1
The 'iswishlist=1' querystring tells the application to put the item in the wishlist rather than the shopping cart.
You'll also have to modify wishlist.aspx to hide the 'Move to Shopping Cart' button.