Unfortunately, this is not allowed. You are to inform us of every website that you use our payment system for. You can do this by creating a separate website profile for each website. As soon as we have approved the website, you receive a new API key from us.
The advantage of this working method is that it's very easy to distinguish between the transactions from the various websites. You can view the statistics per website profile, which makes it easier to enter them into your accounting system.