I assume you want all the speakers active at the same time rather than switching speakers. If that is your goal, you can get a RCA distributor amp which will split a single RCA input into multiple RCA outputs. You than connect the outputs into the inputs of a multi-channel amp.
Google "RCA distributor box" or "RCA distributor amp". You should be able to find many products at different price range and features.
Google "RCA distributor box" or "RCA distributor amp". You should be able to find many products at different price range and features.