Pretty easy to do, but why would you do it? For me it's because I have a work mobile phone, so I basically carry two phone around for the past year. I do not give out my work mobile number to people that is not from my work. Yes, there are some co-worker are friends so they have both numbers, but that is beside the point.
Basically I want to keep my original mobile number that I have for the past 10 years but carry / pay 1 phone bill.
You need to have a google account, http://google.com/voice and it does cost one time transfer fee of $20 (WAIT... but if you are still under contract you may want to wait to avoid early termination fee).
"Get a voice number".
Next screen is select "I want to use my mobile number"
Now google want you to enter your mobile number and it'll verify if it's eligible.
things you need to check off.
now it's time to verify, click on "Call me now"
you'll get a call on the mobile phone number you entered, so google know that you are who you say you are.
time to pay with Wallet.
Now the final screen, then you wait about 3 business days.
Now enjoy all the features in the google voice.