forward proxy for ports 20 and 21

Depending on what you want to do it could literally be as simple as creating a virtual server on those ports. If you have a specific endpoint in mind a standard virtual server with a single pool member would be fine. If you have multiple endpoints, you'd want to use a forwarding virtual server. Be aware that if you are attempting to handle FTP traffic (not an unreasonable expectation when talking about port 21), that you will need to have a listener in place to handle the data channel connection.

If you can give us a bit more detail on what you're trying to accomplish we can probably give you a less general answer.