Why hasn't Luffy asked Sogeking to be his nakama?

Luffy is one who will spontaneously ask talking trees to join his crew, and he seemed impressed by Sogeking, king of the snipers (mysterious as his origins may be). Why didn't he ask Sogeking to join then?

Did he know Sogeking's true identity? Was he not willing to replace Usopp's role as a sniper?