AGA,
Crews don't have that flexibility of building a STAR... a route, yes; but the STAR or SID is NOT! STARs and SIDs are clearly defined arrival/departure routes to regulate the flow of air traffic into or out of an airport. They're constructed by the local regulatory authority to avoid such areas of high terrain, noise sensitive areas, Royal residences... to name but a few, but the crew are more or less legally bound to follow the route as directed by ATC. "Kerbunker" is on the right track, it's likely to be an RNAV SID/STAR, which you cannot follow without the use of an FMC. You're therefore legaly bound by the instruction given to you by ATC. The only exception is if there is an emergency situation that prevents you from doing so; however, a cursory call to ATC to let them know that you aren't following the published SID/STAR is a must.
I mentioned that some of the major carriers do have navigation departments that are authorised by the regulatory authority to build SIDs/STARs, but it's really only a function of an amendment to those already programmed. The larger carriers obviously do have membership to the companies who do provide an amendment service (such as Jeppessen). I mentioned private operators, because just like any private pilot, you can programme your own GPS to carry out much the same job, but the accuracy won't be anywhere near an airline system that will typically have a 'Triple GPS' and 'Triple INS' position update feed to the FMS; nor have the output steering programme to the autopilot system that could follow rapidly changing waypoint database feeds.
It aint simple... and, unless you're only interested in attempting to use a flight simulation programme; don't mess with it in the real world! When ATC give you a STAR arrival; that's what it is... a clear instruction from ATC to fly a defined published route; a route which has been approved by the regulatory authority. An RNAV FMS-based turn between two chart identified points will typically have a dozen or so intermediate waypoints just to navigate you between two chart identified positions... all normally flown by coupling the autopilot to the FMS. It's very accurate when the system is also being updated by DME/DME fixing (or as accurate as it comes without normal VOR/DME stations and being updated by GPS/INS etc). My last airlne that I flew with wouldn't approve the aircraft or crew to follow RNAV approaches; despite our having Triple GPS/INS feed to the FMS. The aircraft equipment must also comply with the state regulatory requirements... just like CAT II, CAT III, CAT IIIA etc, etc; but that's another can of worms to open!
Bedtime for me!