We go through this tutorial by screenshots so it be illustrative than purely theoretical which might mess up, I didn't wanted to spam with too large screenshots, so for preview you'll see medium size image, click on link right below the image to see its full size and observe better.
We consider out 3d model as 175cm, we be lengthening it for 5cm making it as 180cm
1. I chose below 3d model to do this mock-up
https://i.imgur.com/nZB8zQJ2. Duplicate this image into two, so we use one to determine the height and then discard and start using other copy, so duplicate your image into two
3. Crop image from ankles to top of your head, note in image we are not including whole feet we just need from ankle to head top
https://i.imgur.com/jg0u6ZS4. After having right selecting of crop, now crop this image
https://i.imgur.com/pvlgdbG5. Check for size of this image(Top bar -> Image -> canvas size in gimp software), in below image we got height as 348px(pixels) divide this with height 348px/175cm = 1.988571429px as per this each cm in this image represent 1.988571429px
https://i.imgur.com/zDzGaJE6. Discard this cropped image after we have our calculations, we are left with other copy make another duplicate of it, so we have two copies of 3d model
7. Open new layout with extra height than your original image(I choose 520px for this image) copy our 3d model to this layout, now you have it as this, and select half the image(from tibia or femur as per your requirement, here we doing for tibia), we use this selection to separate this 3d model into two pieces
https://i.imgur.com/VPDTMmi8. Cut the select of which we made
https://i.imgur.com/jGoZGnG9. Paste the top part of 3d model, above the bottom part leave the gap in-between
https://i.imgur.com/m6c7SCK10. From previous calculations we know each cm in our image represent 1.988571429px, we need to lengthen 5cm that is 5cm*1.988571429px = 9,942857145px, make a selection of this amount from tibia or femur as per your requirement, in this example we doing it with tibia, we made crop selection for 10px(gimp didn't allow me to use value i calculated so I gone with 10px), as you can see in red marked location(this is gimp software)
https://i.imgur.com/U4IRwkU11. Hit crop we get our lengthened part as below, copy this and paste it into our image from image of step number 9 as third part
https://i.imgur.com/QwK85ej12. Align the third part with bottom part of image, as below
https://i.imgur.com/jxkVWT013. Bring down the top part to join the image as below
https://i.imgur.com/9wexjSXYou have successfully created a mock up increasing 5cm