You can add pets to your wishlist from the
archives. Next to the pet there will be an option to add the pet to your wishlist. Just click that to add it;
However, I'm not sure what you mean by adding pound pets to your wishlist. If you mean adding a pet to your wishlist from a pet's page, you can't do that. However if the pound is open and a pet on the page is on your wishlist it will have a star below it to make it stand out as a wishlist pet. But it won't cause only wishlist pets to show up or anything like that, and you won't be able to add wishlist pets to a trade with the pound, since the trade option is for donations only, so you can't take anything from the pound through a trade.