Hi Beat, I've been considering developing an add-on that allows revenues to be recognized over a period of time because I'm not aware of any add-on that already does this.
Today, CRM assumes that an opportunity's revenues are all recognized when the opportunity is won, but for many industries the accounting standards rule that revenues can only be recognized as the service is delivered.
You can extend CRM to recognize revenues over time -- I've customized it like this for several customers -- but it's quite a bit of effort because revenue recognition accounting rules are often quite complex.