I don't know about Skype or GV, but since I work at Twilio I can answer that for us. You can already port your numbers to Twilio. After creating and upgrading your account, you'll see a port a number option in the dashboard. It takes 7-10 business days.
I should clarify that we can port Google Voice and Skype numbers to Twilio. I meant that I don't know if GV lets you port numbers in yet or not.
It's pretty straightforward. You tell us (I work at Twilio) which numbers you want to port, send us some documentation proving you own them (a copy of your bill and a letter of authorization), we do the rest and get back to you when the port will be complete. It takes about 7-10 business days.