A real simple way to setup an automatic conversion machine with CM is to setup multiple lists where each list has a purpose. Like list 1 could be to sell lead in product and list 2 was about selling a back end product.
This is a very powerful method to automated relationship building with your subscribers. But this of course only makes sense if the subscriber is unsubscribed from list 1 when added to list 2.
So here is +1 vote for what I expect to be a rather simple job for you guys to enable:
Make it possible to choose between subscribing or unsubscribing when creating new feeds.
Here is the the API documention for your convenience: http://www.campaignmonitor.com/api/v2/method/subscriber-unsubscribe/