Normally, if you hold the AF lock button, it will lock the focus on the target you selected when it was still actively focusing in CF mode. If you shoot a few shots, then release the AF lock button, it will be back in CF mode again and you will need to refocus and go through everything again.
If you do this a lot, you might consider changing your camera's settings and work essentially backwards from this scenario. I can't remember the exact setting...seems like it is AF Release. Someone else will help with the details I'm sure. But the concept is to use the AF Lock button to activate the AF focusing so you can set the target focusing point. When you release the AF Lock button, the camera stops focusing and you can recompose to your heart's content. It will not attempt to refocus until you press and hold the AF Lock button again. It takes some time to get used to, but it might be exactly what you are needing.