Well this may or may not work i have never tried this but, as it rolled around in my head it made sense.
1. Generate an IFR flight plan with FSX
2. Copy the plan and paste it into Simroutes flight planning website, be sure you have No sids & No stars showing.
3. Click generate flightplan...When flightplan comes up.
4. Choose aircraft 744 with or w/out winglets.
5. To the left is a box where you change to PMDG
6. Click save flightplan.
7. Go to where you've saved the flightplan, copy and paste in into your PMDG flightplans folder...Write the flightplan down as it appeared when you saved it.
If you were flying to Hong Kong from Tokyo Japan, it would look like this...RJAAVHHH...write this down you will need it later.
8. Go to your aircraft program that..RJAAVHHH..into your FMC as Coroute, the entire flightplan will be entered into the legs page.
9. Choose your SID and runway taxi to the active, get your clearance and hit the skys IFR.
FSX will follow it's own flightplans but, won't follow flightplans generated from other planning sites it seems.
This may or may not work like i said so, don't hold me to this...I'm going to try it out my self tonight so, i'm hoping it works for my sake as well. B)