I'm not exactly sure what resolution you are looking for. Is it a) the BIG-IP should connect to the content server using the same schema (HTTP or HTTPS) the client used, or b) the BIG-IP should connect to the webserver via HTTP, but signal the schema the client used when connecting?
If what you need is 'a', then you need to place a server-ssl profile on the VS handling your HTTPS traffic, to allow the BIG-IP to re-encrypt the traffic. You will also need to configure your HTTPS VS with a pool that contains member instances that are listening for HTTPS traffic.
If what you need is 'b', then you will need to use a simple iRule to look at the service the client was trying to reach ( TCP::local_port would do this), and discriminate between HTTP and HTTPS traffic. Once you know the service, you can insert a header into the client request using HTTP::header, with a value of HTTP or HTTPS as determined by the local port.
Hope that helps.