iRule to add Remote address in TCP header

Using TCP::payload replace may be a solution, but as Uni points out it depends on the application, its data structure, and when and where you want to insert the new data. That said, in the truest sense, the TCP::payload command really only works on payload and not actual TCP headers. To get to the TCP headers, the options fields specifically, you could use the TCP::options set command. Again though, this is all rather dependent on the data structure. If you could give us some more information it might help.