Aweber: Give existing subscribers option to receive more regular emails

Hi all,
I've tried searching, but haven't found what I'm looking for. If I've missed it, apologies, and please post a link! I'm using Aweber.

I have a list that was marketed for a long time as a monthly newsletter. Some of my subscribers only want to receive monthly emails (and that is what they signed up for), but some want to receive more regular emails (I'm thinking every second week, plus the original monthly email, bringing up to 3 emails per month).

For the two-weekly email, I'd like to take advantage of Aweber's auto Follow Up emails, and create a 'course' of sorts.

I'm just not sure how to get people into this second group. I was thinking segments on the existing list, however a) I don't know how to give subscribers the option to join this new segment and b) I don't think I can send the auto Follow Ups to a segment of a list.

So now I'm thinking it will have to be a separate list. How do I get subscribers onto this second list? Send out an auto email to the original list (say a few days after the original emails welcoming them to the list), inviting them to sign up to the two-weekly "course" emails? Link this to a sign up form? But then subscribers will need to fill out their details (name/email) again... Is this the best way to deal with this, or am I overlooking something?

Thanks for any advice!