Yes. You can port Google Voice numbers to Twilio as long as they are 10-digit US numbers.
In order to port a number away from Google Voice you must unlock the numbers through your Google account.
Instead of an official billing document, you must submit a screenshot of the page that confirms you have unlocked the numbers in your Google account. Please take a screenshot of the Google Voice settings page making sure the unlocked phone numbers and your email address appear in the screenshot.
It's important to confirm that the first name, last name, and address you provide on the Letter of Authorization match those in your account profile exactly.
To begin porting your number, please visit the Porting page in your Account Portal and enter the information regarding the numbers you'd like to port.