That’s possible but that’s not good idea! Molex may not be able to handle that high amps. If you have good PSU (Not no-name, and it should be at least 550+W to be safe) and you PC is not too power hungry you may be able to charge and use PC at same time but I wouldn’t suggest that. Still, if you want to try that then at least use multiple molex`es. You can use adapter like this 
Use at least two molex that is NOT on same wire but come on different wires from PSU.
There are multiple videos that show how to do that.
Here is nice one at the end just connect wire to PSU. Of course there are easier methods to do that than shown in video. That video will help to get idea and you can do that in your own way.