Is it possible to manipulate the content of an audience?

I want to add and remive customers to and from an AdWords remarketing audience via the API. Until now I am able to retrieve the list and can access its field. But neither am I able to access the elemnets of the list, nor can I add another one to it.

Is there any to way to achieve the above?

Not directly with AdWords.

The reason is that AdWords needs to have seen the doubleclick.net cookie to add a user's ID to the audience list, and you wont be able to see the cookies for doubleclick.net (or at least the browsers should not let you see it anyway) so you have no ID to add.

However, depending on what you are trying to do there are a few options:

1. If the users have already been added to another AdWords audience and you want to put people into different audiences depending on what they've done (e.g. put high spenders in one audience list, low spenders in another etc) you can use the AdWords audience rule builder in the UI to have AdWords do the processing for you.
2. You could use Google Analytics and user data upload to upload extra information about the users, then use GA's audience list builder to make rules on that extra information, then push those lists to AdWords.

Hope that helps.