Simple answer is you can't because <render:satellitepage /> inserts a Satellite Server pagelet, which is not executed by ContentServer.
You can use ics.ReadPage(pagename, args) which will return a String after calling a SatelliteServer page however this makes and extra http request.
I would asl why you feel like you need to do this? What are you trying to achieve? Maybe there is an alternative solution for you.
What are you really trying to do? If you supply more info about your actual use case it would help