Hey Wladis, seeing that ur interested in doing this kind of stuff im telling the steps that you must go through:
-Step 1: You will need either a copy of KH1 or KH2. Download a ps2 emulator and then make an ISO file of the KH1 ( or KH2) CD that you have.
-Step 2: You have to find the khdump.exe ( for KH1 ) or the khdump2.exe ( for KH2 ) to rip off the models from the ISO file. You can google it and download it ( at your own risk, since this is illegal )
-Step 3:Put the Khdump.exe(or khdump2.exe) and the ISO file into a same foldier. Copy the Kingdom.idx file ( located inside the ISO file you make ) to this same folder.
-Step 4: Drag the ISO file and the Kingdom.idx into the Khdump.exe( or khdump2.exe). It will show a bunch of mdls ( or mdlx ) files being dumped. You will find these files in a folder nearby ( I found them in C:\Document and Settings )
-Step 5:You need to download the model viewer, which is called ModelViewerWXD3D9_0.3. Download this at your risk, since it is illegal. ( hint : there is one on filefront )
-Step 6:Find the model you want to rip. Download 3DRipper Dx
www.deep-shadows.com/hax/3DRipperDX.htm . Launch the ModelViewer using 3D Ripper Dx. Open the model you want to rip. Hit the capture key ( F12 ) so 3D Ripper Dx can rip off the model for you. Make sure before hitting F12 you have checked the square "also converted to wave front .OBJ". You will a OBJ file and a bunch of TEX files
-Step 7:Download Mirex's Biturn. Its on the wiki system of qhimm. Using biturn, open the .OBJ file you just ripped. Convert it into a 3DS file.
-Step 8: Now you will need 3D Max to edit this 3DS file. This is tricky, this 3D Max program costs you $90. However, you can try the free trial version of it. After you get 3D Max, import the 3DS file into the program. You have to remove the parts of the model that you dont want and also you have to rebuild some of the parts. Fore more info, read squeeble's thread :
http://forums.qhimm.com/index.php?topic=6576.0-Step 9: Download PCreator. Its on qhimm wiki. Load all the 3DS files into PCreator and apply the uv chords to these files so that the TEX files can apply textures to these 3DS files. If the texture works perfect. Save the 3DS files as P files. For more info , read this thread
http://forums.qhimm.com/index.php?topic=6624.msg82833#msg82833-Step 10: Download Lgptools. You have to decompile the char.lgp and the battle.lgp file in your ff7 folder and extract them into a separate folder
-Step 10: Download Borde's Kimera program. You have to find the model files of the characters you want to replace ( for example Cloud, his battle model is rtaa ). Replace Cloud's parts with the P files you have just converted. You will fail many times, because the texture may not load correctly.
-Step 11:Adjust the position of these parts so that they fit with the skeleton and animations. After you are done, use lgptools to make a new battle.lgp ( or char.lgp ) and test your new model in game.
So, to my conclusion, this is way beyond knowledge of a person who isnt a pro at computers. Also, it costs you $20 for KH copy and $90 for 3d Max. I use the free trial version of 3d Max, but it only lasts for 15 days. I got to step 10, with cloud's head from KH1 but the texture never applied correctly. Then my 3d max expires. It sucks, doesn't it? But in the meantime, I find out that you can make your models look like the KH models by using just Kimera and Paint. Kudos to Borde because his program is awesome, free and much easier to use than the 3D Max. You can edit all the parts from the original using kimera ( vertex colour editing, create new polygon, adjusting position and size ) and paint ( to edit the texture ). Here are my results ( of course, it isnt as good as ripping the models from KH, but still looks ok ):

So you can either follow the 11 steps or just do something similar to what i ve just come up with. Good luck.