You can splice in extra length, no problem. Soldering and heat shrink tubing is the best way to go. Make sure that you don't swap positive and negative. Timing/phase issues? No, electricity travels at nearly the speed of light (186,000 miles/sec) in copper!
A popular trick is to splice the two ends of the speaker wire together, shrink wrap the positive and negative individually, wrap them individually in electrical tape, then wrap them together with electrical tape. Doing it this way will prevent oxidation while still providing you with a clean splice to pass your signal across to the speakers.
Timing issues are not really a problem with the things that you would listen to in 5.1 surround sound, only with music listening.
Yes, but I wouldn't just twist more wire onto the original piece though. It is better if it is all in one piece. I would buy some cheap wire from Radio Shack or Walmart. They use to sell 50ft for around $10-15. And yes, if you do make the wires REALLY long --say over 100ft you can get some delays and problems.