OEM crossbars require popping off the plastic end caps and sliding them in to the channel on the factory rails. Your width is then of course limited to the width of the factory bar/rail system. If you're hoping to haul multiple toys (2 kayaks, box and bikes, etc.) you may not have enough real estate. The slight curve of the factory bars is also somewhat odd to work with IMHO.
Rack systems from
Thule and
Yakima (for example) require two primary parts for the base system. One part clamps on to the rails and allows for any length bar (the 2nd part)...either round/square or more current aero options. Check out the Yakima Railgrab or Timberline options, and the Crossroads or Aeroblade systems from Thule. There are others, these are probably the most common for 'bar only' style roof racks.
Hope that helps.