Your question isn't very clear, but if you know a bit about electronics, then yes, you can alter the length of any cable you want. I suspect however that you don't, in which case, get someone who knows to do it for you, or just leave well alone. Even with the computer turned off and unplugged from the wall, there is still enough current in the PSU capacitors to kill or seriously hurt you. I suggest if you didn't know this already to steer well clear.
Power supply cables - no problem. USB cable, make sure you use the same style of cable. HDD cable, you can buy longer ones, this is a better option than trying to extend the one you have. There are some signal issues if you try to extend the cable.
Only if you know what you are doing (if you have experience with this kind of stuff). If you get the cables wrong you can fry your power supply, your motherboard, usb, or HDD. If you know what you are doing, then it is fine. It would be best to solder in the extensions, but not necessary. What I do is for each end of the cables that I will join, I divide the individual wires into 2 parts, and then intertwine them with the other cable's wires. Then I individually tape each cable, and finally tape all the cables. *Note: by cable i mean one the black, red, or yellow ones. By wire i mean each individual strand of metal in each cable.
No chance in hell. besides the fact you would have to extend the wires in the data cables with the EXACTLY same lenght, the chances to break the shielding is 100%. Not mentioning the fact that the power cables do not support the extra load given by makeshift connectors.
hahah are you kidding...yeah if you want to die...if you did that your psu would destroy your entire system when and if you tried starting it up and would most likely start a fire somehow...PSU's are very very sensitive and in no way should you try altering them..it would be a very very bad thing to do...best thing to do is buy a new one and make sure the cables are long enough
