Have you tried doing a Web Search with "myRIO and RS-485"? This question has also been asked here on the LabVIEW Forums. This is definitely not "built-in", but can be done, with a reasonable (?) amount of effort. Note that you may also need some digital circuitry to change the TTL levels from the myRIO's DIO lines to the pulse levels specified in the RS-485 standard. It means "programming to the RS-485 standard". I didn't do an exhaustive Web search -- it is possible that you might find a good desciption of "how to do this" (or, even better, find someone who has built a circuit or a chip to do the translation for you ...).
Bob Schor