1. You can get an .ODR by using OpenIV and opening playerped.rpf, then right click and export a file as open format .ODR, I used lowr_000_u.wdr and exported as lowr_000_u.odr.
2. Use the 3DS Max and the OFIO Script to import a body part from Niko (.ODR file) by clicking on DrawableODR Import Single button.
3. Use OpenIV to open anim.img and export the chosen animation file as an .OAD file by right clicking and selecting the Export to openFormat option (.oad). Inside the folder you export will be .ONIM files.
4. Select the Char bone in 3DS Max, import the .ONIM animation file you want to modify by clicking on AnimationONIM Import Single button.
5. Use the rotate tool to rotate bones that are keyed. If you don't see key frames, don't rotate it.
6. Select the Char bone when finished and Export Skel under /Animation/ONIM.
7. Rename exported ONIM file to match the animation you are replacing, example is rename lowr_000_u.onim to free_fall_fast.onim.
8. Replace animation file in the folder you extracted from OpenIV containing your OAD file.
9. Use OpenIV and open anim.img, click EDIT MODE button, then click NewImport openFormats and select the .OAD file and import.
10. Run GTA IV and look how amazing your new animations look! You can also use openIV's Animation Viewer.
-Credit goes to H1Vltg3 and the Iron Man IV mod team