Modification

Basically I have WHMCS to handle all payments.

I want to make the 'Add to Cart' on the Memberships area take the user to my WHMCS order form...

How can I do this?

For the member bar do it here http://www.yoursite.net/administration/member_menu_compose.php?menu_position=top_extra

As for the add to cart you might be better off doing a iframe into the site and changing the link in the main menu. But you will have to setup the users membership your self.

I only want to do it for memberships so is there a way I can change the add to cart button on the membership page so it redirects them to WHMCS?

push

The best way is to interface with WHMCS using the API:

http://wiki.whmcs.com/API:Functions

We have done a bunch of WHMCS modifications before, using the API - it works well.

But not to handle the Dolphin Memberships... the idea is interesting.

-Smoge