Boeing247, do you know how to file an IFR flight plan within FSX and fly under the built-in ATC yet?  Once you know how to do that, you can use the built in GPS system to fly your flight plan either manually or with the autopilot.  

You can actually do this without any mods by simply assigning a key combination to toggle NAV/GPS and then use NAV Hold with the 727 autopilot.  There is a thread on how to add a GPS/NAV switch to your 2D autopilot panel as well, to make things easier.  

The built-in ATC will vector you to line up with your flight plan, but you have to navigate from there.  Using the GPS system and the autopilot to do this is pretty much the same thing that an INS system does.  Only with the INS system you have to manually plug in all your waypoints.  The GPS will already have all of that ready to go for you when you have an IFR filght plan.