No you can't. I had a LONG discussion about this with one of the airlines on the same issue. Essentially PHL-ORD-MSN roundtrip was $212 where PHL-ORD was $300 but used the same plane for the first leg of the trip. I wanted to buy the ticket to MSN and then just not use it. But he said it would void my return ticket. He gave me a long involved reasoning that made no sense (its a contract... blah blah blah).

I am firmly convinced that airlines make it up as they go along.