The FS2004 world is a cylinder, when you get close enough to either pole you "warp" to the corresponding point 180deg across the pole, heading in the opposite direction. I believe this is somewhere around the 10th, maybe 5th, parallel; so we cannot recreate a polar flight.
For the OP, some FS2002 scenery will work in FS2004, but the farther from the equator it is the less likely it will work well, and it is doubtful it will work at all in FSX (coordinates and elevations will be too far off, it may appear but not where expected, runways may not be properly oriented, navaids could be incorrectly placed ...issues with the latter can even cause CTDs).
"Conversion" as such is doubtful, you could use all the scenery objects to create a new scenery just for FSX.