Yes. You can look at your favorite website as a series of one-ways. Either do a printout or write down options which make sense to you and then go back and book the thing as a RT or Multi-city depending on how it all works out.

One caution -- if you don't have experience, it's well worth the telephone booking fee to call your originating airline or the fee for a TA. Some third party sites create multiple itineraries. If you misconnect, you are nobody's responsibility but your own.