Mailchimp add or update

Hi,

I've got a site where there are multiple forms in different places to add a user to the client's mailchimp subscriber list, setting different interests in each place. Mailchimp recommend a single list for some really good reasons, but if I have a user who's signed up via (e.g.) the newsletter page and then add themselves via the (e.g.) magic users page, they aren't being added to any list because they are already on the main list and just need their interests updating.

Is there any way that I can do this? It looks like the Mailchimp API has an add or update call rather than just an add (Mailchimp API documentation).